[PATCH v5 5/5] mm/filemap: Allow arch to request folio size for exec memory
Tao Xu
tao.xu at arm.com
Fri Jul 11 08:41:11 PDT 2025
On 09/06/2025 10:27, Ryan Roberts wrote:
> Change the readahead config so that if it is being requested for an
> executable mapping, do a synchronous read into a set of folios with an
> arch-specified order and in a naturally aligned manner. We no longer
> center the read on the faulting page but simply align it down to the
> previous natural boundary. Additionally, we don't bother with an
> asynchronous part.
>
> On arm64 if memory is physically contiguous and naturally aligned to the
> "contpte" size, we can use contpte mappings, which improves utilization
> of the TLB. When paired with the "multi-size THP" feature, this works
> well to reduce dTLB pressure. However iTLB pressure is still high due to
> executable mappings having a low likelihood of being in the required
> folio size and mapping alignment, even when the filesystem supports
> readahead into large folios (e.g. XFS).
>
> The reason for the low likelihood is that the current readahead
> algorithm starts with an order-0 folio and increases the folio order by
> 2 every time the readahead mark is hit. But most executable memory tends
> to be accessed randomly and so the readahead mark is rarely hit and most
> executable folios remain order-0.
>
> So let's special-case the read(ahead) logic for executable mappings. The
> trade-off is performance improvement (due to more efficient storage of
> the translations in iTLB) vs potential for making reclaim more difficult
> (due to the folios being larger so if a part of the folio is hot the
> whole thing is considered hot). But executable memory is a small portion
> of the overall system memory so I doubt this will even register from a
> reclaim perspective.
>
> I've chosen 64K folio size for arm64 which benefits both the 4K and 16K
> base page size configs. Crucially the same amount of data is still read
> (usually 128K) so I'm not expecting any read amplification issues. I
> don't anticipate any write amplification because text is always RO.
>
> Note that the text region of an ELF file could be populated into the
> page cache for other reasons than taking a fault in a mmapped area. The
> most common case is due to the loader read()ing the header which can be
> shared with the beginning of text. So some text will still remain in
> small folios, but this simple, best effort change provides good
> performance improvements as is.
>
> Confine this special-case approach to the bounds of the VMA. This
> prevents wasting memory for any padding that might exist in the file
> between sections. Previously the padding would have been contained in
> order-0 folios and would be easy to reclaim. But now it would be part of
> a larger folio so more difficult to reclaim. Solve this by simply not
> reading it into memory in the first place.
>
> Benchmarking
> ============
>
> The below shows pgbench and redis benchmarks on Graviton3 arm64 system.
>
> First, confirmation that this patch causes more text to be contained in
> 64K folios:
>
> +----------------------+---------------+---------------+---------------+
> | File-backed folios by| system boot | pgbench | redis |
> | size as percentage of+-------+-------+-------+-------+-------+-------+
> | all mapped text mem |before | after |before | after |before | after |
> +======================+=======+=======+=======+=======+=======+=======+
> | base-page-4kB | 78% | 30% | 78% | 11% | 73% | 14% |
> | thp-aligned-8kB | 1% | 0% | 0% | 0% | 1% | 0% |
> | thp-aligned-16kB | 17% | 4% | 17% | 3% | 20% | 4% |
> | thp-aligned-32kB | 1% | 1% | 1% | 2% | 1% | 1% |
> | thp-aligned-64kB | 3% | 63% | 3% | 81% | 4% | 77% |
> | thp-aligned-128kB | 0% | 1% | 1% | 1% | 1% | 2% |
> | thp-unaligned-64kB | 0% | 0% | 0% | 1% | 0% | 1% |
> | thp-unaligned-128kB | 0% | 1% | 0% | 0% | 0% | 0% |
> | thp-partial | 0% | 0% | 0% | 1% | 0% | 1% |
> +----------------------+-------+-------+-------+-------+-------+-------+
> | cont-aligned-64kB | 4% | 65% | 4% | 83% | 6% | 79% |
> +----------------------+-------+-------+-------+-------+-------+-------+
>
> The above shows that for both workloads (each isolated with cgroups) as
> well as the general system state after boot, the amount of text backed
> by 4K and 16K folios reduces and the amount backed by 64K folios
> increases significantly. And the amount of text that is contpte-mapped
> significantly increases (see last row).
>
> And this is reflected in performance improvement. "(I)" indicates a
> statistically significant improvement. Note TPS and Reqs/sec are rates
> so bigger is better, ms is time so smaller is better:
>
> +-------------+-------------------------------------------+------------+
> | Benchmark | Result Class | Improvemnt |
> +=============+===========================================+============+
> | pts/pgbench | Scale: 1 Clients: 1 RO (TPS) | (I) 3.47% |
> | | Scale: 1 Clients: 1 RO - Latency (ms) | -2.88% |
> | | Scale: 1 Clients: 250 RO (TPS) | (I) 5.02% |
> | | Scale: 1 Clients: 250 RO - Latency (ms) | (I) -4.79% |
> | | Scale: 1 Clients: 1000 RO (TPS) | (I) 6.16% |
> | | Scale: 1 Clients: 1000 RO - Latency (ms) | (I) -5.82% |
> | | Scale: 100 Clients: 1 RO (TPS) | 2.51% |
> | | Scale: 100 Clients: 1 RO - Latency (ms) | -3.51% |
> | | Scale: 100 Clients: 250 RO (TPS) | (I) 4.75% |
> | | Scale: 100 Clients: 250 RO - Latency (ms) | (I) -4.44% |
> | | Scale: 100 Clients: 1000 RO (TPS) | (I) 6.34% |
> | | Scale: 100 Clients: 1000 RO - Latency (ms)| (I) -5.95% |
> +-------------+-------------------------------------------+------------+
> | pts/redis | Test: GET Connections: 50 (Reqs/sec) | (I) 3.20% |
> | | Test: GET Connections: 1000 (Reqs/sec) | (I) 2.55% |
> | | Test: LPOP Connections: 50 (Reqs/sec) | (I) 4.59% |
> | | Test: LPOP Connections: 1000 (Reqs/sec) | (I) 4.81% |
> | | Test: LPUSH Connections: 50 (Reqs/sec) | (I) 5.31% |
> | | Test: LPUSH Connections: 1000 (Reqs/sec) | (I) 4.36% |
> | | Test: SADD Connections: 50 (Reqs/sec) | (I) 2.64% |
> | | Test: SADD Connections: 1000 (Reqs/sec) | (I) 4.15% |
> | | Test: SET Connections: 50 (Reqs/sec) | (I) 3.11% |
> | | Test: SET Connections: 1000 (Reqs/sec) | (I) 3.36% |
> +-------------+-------------------------------------------+------------+
>
> Reviewed-by: Jan Kara <jack at suse.cz>
> Acked-by: Will Deacon <will at kernel.org>
> Signed-off-by: Ryan Roberts <ryan.roberts at arm.com>
Tested-by: Tao Xu <tao.xu at arm.com>
Observed similar performance optimization and iTLB benefits in mysql
sysbench on Azure Cobalt-100 arm64 system.
Below shows more .text sections are now backed by 64K folios for the
52MiB mysqld binary file in XFS, and more in 128K folios when increasing
the p_align from default 64k to 2M in ELF header:
+----------------------+-------+-------+-------+
| | mysql |
+----------------------+-------+-------+-------+
| |before | after |
+----------------------+-------+-------+-------+
| | | p_align |
| | | 64k | 2M |
+----------------------+-------+-------+-------+
| thp-aligned-8kB | 1% | 0% | 0% |
| thp-aligned-16kB | 53% | 0% | 0% |
| thp-aligned-32kB | 0% | 0% | 0% |
| thp-aligned-64kB | 3% | 72% | 1% |
| thp-aligned-128kB | 0% | 0% | 67% |
| thp-partial | 0% | 0% | 5% |
+----------------------+-------+-------+-------+
The resulting performance improvment is +5.65% in TPS throughput and
-6.06% in average latency, using 16 local sysbench clients to the mysqld
running on 32 cores and 12GiB innodb_buffer_pool_size. Corresponding
iTLB effectiveness benefits can also be observed from perf PMU metrics:
+-------------+--------------------------+------------+
| Benchmark | Result | Improvemnt |
+=============+==========================+============+
| sysbench | TPS | 5.65% |
| | Latency (ms)| -6.06% |
+-------------+--------------------------+------------+
| perf PMU | l1i_tlb (M/sec)| +1.11% |
| | l2d_tlb (M/sec)| -13.01% |
| | l1i_tlb_refill (K/sec)| -46.50% |
| | itlb_walk (K/sec)| -64.03% |
| | l2d_tlb_refill (K/sec)| -33.90% |
| | l1d_tlb (M/sec)| +1.24% |
| | l1d_tlb_refill (M/sec)| +2.23% |
| | dtlb_walk (K/sec)| -20.69% |
| | IPC | +1.85% |
+-------------+--------------------------+------------+
> ---
> arch/arm64/include/asm/pgtable.h | 8 ++++++
> include/linux/pgtable.h | 11 ++++++++
> mm/filemap.c | 47 ++++++++++++++++++++++++++------
> 3 files changed, 57 insertions(+), 9 deletions(-)
>
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 88db8a0c0b37..7a7dfdce14b8 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -1643,6 +1643,14 @@ static inline void update_mmu_cache_range(struct vm_fault *vmf,
> */
> #define arch_wants_old_prefaulted_pte cpu_has_hw_af
>
> +/*
> + * Request exec memory is read into pagecache in at least 64K folios. This size
> + * can be contpte-mapped when 4K base pages are in use (16 pages into 1 iTLB
> + * entry), and HPA can coalesce it (4 pages into 1 TLB entry) when 16K base
> + * pages are in use.
> + */
> +#define exec_folio_order() ilog2(SZ_64K >> PAGE_SHIFT)
> +
> static inline bool pud_sect_supported(void)
> {
> return PAGE_SIZE == SZ_4K;
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 0b6e1f781d86..e4a3895c043b 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -456,6 +456,17 @@ static inline bool arch_has_hw_pte_young(void)
> }
> #endif
>
> +#ifndef exec_folio_order
> +/*
> + * Returns preferred minimum folio order for executable file-backed memory. Must
> + * be in range [0, PMD_ORDER). Default to order-0.
> + */
> +static inline unsigned int exec_folio_order(void)
> +{
> + return 0;
> +}
> +#endif
> +
> #ifndef arch_check_zapped_pte
> static inline void arch_check_zapped_pte(struct vm_area_struct *vma,
> pte_t pte)
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 4b5c8d69f04c..93fbc2ef232a 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3238,8 +3238,11 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
> }
> #endif
>
> - /* If we don't want any read-ahead, don't bother */
> - if (vm_flags & VM_RAND_READ)
> + /*
> + * If we don't want any read-ahead, don't bother. VM_EXEC case below is
> + * already intended for random access.
> + */
> + if ((vm_flags & (VM_RAND_READ | VM_EXEC)) == VM_RAND_READ)
> return fpin;
> if (!ra->ra_pages)
> return fpin;
> @@ -3262,14 +3265,40 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
> if (mmap_miss > MMAP_LOTSAMISS)
> return fpin;
>
> - /*
> - * mmap read-around
> - */
> fpin = maybe_unlock_mmap_for_io(vmf, fpin);
> - ra->start = max_t(long, 0, vmf->pgoff - ra->ra_pages / 2);
> - ra->size = ra->ra_pages;
> - ra->async_size = ra->ra_pages / 4;
> - ra->order = 0;
> + if (vm_flags & VM_EXEC) {
> + /*
> + * Allow arch to request a preferred minimum folio order for
> + * executable memory. This can often be beneficial to
> + * performance if (e.g.) arm64 can contpte-map the folio.
> + * Executable memory rarely benefits from readahead, due to its
> + * random access nature, so set async_size to 0.
> + *
> + * Limit to the boundaries of the VMA to avoid reading in any
> + * pad that might exist between sections, which would be a waste
> + * of memory.
> + */
> + struct vm_area_struct *vma = vmf->vma;
> + unsigned long start = vma->vm_pgoff;
> + unsigned long end = start + ((vma->vm_end - vma->vm_start) >> PAGE_SHIFT);
> + unsigned long ra_end;
> +
> + ra->order = exec_folio_order();
> + ra->start = round_down(vmf->pgoff, 1UL << ra->order);
> + ra->start = max(ra->start, start);
> + ra_end = round_up(ra->start + ra->ra_pages, 1UL << ra->order);
> + ra_end = min(ra_end, end);
> + ra->size = ra_end - ra->start;
> + ra->async_size = 0;
> + } else {
> + /*
> + * mmap read-around
> + */
> + ra->start = max_t(long, 0, vmf->pgoff - ra->ra_pages / 2);
> + ra->size = ra->ra_pages;
> + ra->async_size = ra->ra_pages / 4;
> + ra->order = 0;
> + }
> ractl._index = ra->start;
> page_cache_ra_order(&ractl, ra);
> return fpin;
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