[PATCH v3 01/40] mm/vma: fix mmap_prepare file handling, remove file_doesnt_need_get
Liam R. Howlett
liam at infradead.org
Thu Sep 24 12:00:44 PDT 2026
On 26/09/17 05:22PM, Lorenzo Stoakes (ARM) wrote:
> The map->file_doesnt_need_get flag is confusing and the existing
> implementation has holes.
>
> Drivers are permitted to change the owning file of a mapping. If they do
> so, they are required to take a reference on that file.
>
> The mmap() operation which ultimately invokes __mmap_region() is guaranteed
> to drop the refcount for the original file the mapping was made under, but
> this is not true for the replaced file.
>
> This has been addressed so far by tracking map->file_doesnt_need_get, which
> is rather poorly named and unfortunately fails to correctly track whether
> or not an additional put were needed in a number of cases.
>
> Make life easier by removing this flag, and instead drop the reference for
> both mmap_prepare and the deprecated mmap callback in a new function
> put_map().
>
> Track whether this needs to be done by aligning mmap_state with
> vm_area_desc and store the original file in the map->file field, keeping
> the updated file in map->vm_file.
>
> In order to have the same behaviour for both types of hooks, only drop the
> reference __mmap_new_file_vma() itself took in its error path, deferring
> the replaced file's reference to put_map().
>
> To make this work correctly, map->vm_file has to be updated before any
> error handling, so update __mmap_new_file_vma() and call_mmap_prepare() to
> set this field first.
>
> Also when mmap_prepare() changes the file and is then merged, the reference
> count also must be decremented, so update the logic to call put_map() in
> this case too.
>
> Also update __compat_vma_mmap() to manually perform this step for stacked
> file systems using the compatibility layer, and update
> compat_set_vma_from_desc() to replace vma_set_file() with a correct
> refcount/file update.
>
> No in-tree driver is impacted by the incorrect implementation of this
> currently (no driver that does this is mergeable for one), so this does not
> need to be a fix.
>
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs at kernel.org>
> ---
> mm/internal.h | 1 +
> mm/util.c | 5 +++-
> mm/vma.c | 83 +++++++++++++++++++++++++++++++++++------------------------
> mm/vma.h | 6 +++--
> 4 files changed, 59 insertions(+), 36 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 0dca33db068f..fe576d468af4 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -7,6 +7,7 @@
> #ifndef __MM_INTERNAL_H
> #define __MM_INTERNAL_H
>
> +#include <linux/file.h>
> #include <linux/fs.h>
> #include <linux/khugepaged.h>
> #include <linux/mm.h>
> diff --git a/mm/util.c b/mm/util.c
> index bf0513d1d3d0..016932780925 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -1228,8 +1228,11 @@ int __compat_vma_mmap(struct vm_area_desc *desc,
>
> /* Perform any preparatory tasks for mmap action. */
> err = mmap_action_prepare(desc);
> - if (err)
> + if (err) {
> + if (desc->vm_file != vma->vm_file)
> + fput(desc->vm_file);
> return err;
> + }
> /* Update the VMA from the descriptor. */
> compat_set_vma_from_desc(vma, desc);
> /* Complete any specified mmap actions. */
> diff --git a/mm/vma.c b/mm/vma.c
> index 55917d097933..fa784f069da4 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -24,7 +24,8 @@ struct mmap_state {
> vm_flags_t vm_flags;
> vma_flags_t vma_flags;
> };
> - struct file *file;
> + struct file *file; /* mmap()-specified file. */
I guess renaming file to mmaped_file would be a lot more changes.
> + struct file *vm_file; /* May be updated by mmap_prepare. */
> pgprot_t page_prot;
>
> /* User-defined fields, perhaps updated by .mmap_prepare(). */
> @@ -43,8 +44,6 @@ struct mmap_state {
>
> /* Determine if we can check KSM flags early in mmap() logic. */
> bool check_ksm_early :1;
> - /* If .mmap_prepare changed the file, we don't need to pin. */
> - bool file_doesnt_need_get :1;
> };
>
> #define MMAP_STATE(name, mm_, vmi_, addr_, len_, pgoff_, anon_pgoff_, vma_flags_, file_) \
> @@ -58,6 +57,7 @@ struct mmap_state {
> .pglen = PHYS_PFN(len_), \
> .vma_flags = vma_flags_, \
> .file = file_, \
> + .vm_file = file_, \
> .page_prot = vma_flags_to_page_prot(vma_flags_), \
> }
>
> @@ -70,7 +70,7 @@ struct mmap_state {
> .vma_flags = (map_)->vma_flags, \
> .pgoff = (map_)->pgoff, \
> .anon_pgoff = (map_)->anon_pgoff, \
> - .file = (map_)->file, \
> + .file = (map_)->vm_file, \
> .prev = (map_)->prev, \
> .middle = vma_, \
> .next = (vma_) ? NULL : (map_)->next, \
> @@ -2447,7 +2447,7 @@ void mm_drop_all_locks(struct mm_struct *mm)
> */
> static bool accountable_mapping(struct mmap_state *map)
> {
> - const struct file *file = map->file;
> + const struct file *file = map->vm_file;
>
> /*
> * hugetlb has its own accounting separate from the core VM
> @@ -2496,7 +2496,7 @@ static void vms_abort_munmap_vmas(struct vma_munmap_struct *vms,
>
> static void update_ksm_flags(struct mmap_state *map)
> {
> - map->vma_flags = ksm_vma_flags(map->mm, map->file, map->vma_flags);
> + map->vma_flags = ksm_vma_flags(map->mm, map->vm_file, map->vma_flags);
> }
>
> static void set_desc_from_map(struct vm_area_desc *desc,
> @@ -2506,7 +2506,7 @@ static void set_desc_from_map(struct vm_area_desc *desc,
> desc->end = map->end;
>
> desc->pgoff = map->pgoff;
> - desc->vm_file = map->file;
> + desc->vm_file = map->vm_file;
> desc->vma_flags = map->vma_flags;
> desc->page_prot = map->page_prot;
> }
> @@ -2586,6 +2586,10 @@ static int __mmap_setup(struct mmap_state *map, struct vm_area_desc *desc,
> return 0;
> }
>
> +static bool map_same_file(struct mmap_state *map)
> +{
> + return map->vm_file == map->file;
> +}
>
> static int __mmap_new_file_vma(struct mmap_state *map,
> struct vm_area_struct *vma)
> @@ -2593,20 +2597,23 @@ static int __mmap_new_file_vma(struct mmap_state *map,
> struct vma_iterator *vmi = map->vmi;
> int error;
>
> - vma->vm_file = map->file;
> - if (!map->file_doesnt_need_get)
> - get_file(map->file);
> + vma->vm_file = map->vm_file;
> + if (map_same_file(map))
> + get_file(map->vm_file);
>
> - if (!map->file->f_op->mmap)
> + if (!map->vm_file->f_op->mmap)
> return 0;
>
> error = mmap_file(vma->vm_file, vma);
> + map->vm_file = vma->vm_file;
> +
You set vma->vm_file to map->vm_file unconditionally above, is this
necessary?
> if (error) {
> UNMAP_STATE(unmap, vmi, vma, vma->vm_start, vma->vm_end,
> map->prev, map->next);
> - fput(vma->vm_file);
> - vma->vm_file = NULL;
> + if (map_same_file(map))
> + fput(map->vm_file);
>
> + vma->vm_file = NULL;
> vma_iter_set(vmi, vma->vm_end);
> /* Undo any partial mapping done by a device driver. */
> unmap_region(&unmap);
> @@ -2623,7 +2630,6 @@ static int __mmap_new_file_vma(struct mmap_state *map,
> !vma_flags_test(&map->vma_flags, VMA_MAYWRITE_BIT) &&
> vma_test(vma, VMA_MAYWRITE_BIT));
>
> - map->file = vma->vm_file;
> map->vma_flags = vma->flags;
>
> return 0;
> @@ -2631,7 +2637,7 @@ static int __mmap_new_file_vma(struct mmap_state *map,
>
> static void map_set_anon(struct mmap_state *map)
> {
> - map->file = NULL;
> + map->vm_file = NULL;
> map->vm_ops = NULL;
> map->pgoff = map->addr >> PAGE_SHIFT;
> }
> @@ -2643,7 +2649,7 @@ static bool map_is_private(const struct mmap_state *map)
>
> static bool map_is_anon(const struct mmap_state *map)
> {
> - return map_is_private(map) && !map->file;
> + return map_is_private(map) && !map->vm_file;
> }
>
> /*
> @@ -2688,7 +2694,7 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap,
> }
>
> /* Invoke callbacks. */
> - if (map->file)
> + if (map->vm_file)
> error = __mmap_new_file_vma(map, vma);
> else if (!is_anon)
> error = shmem_zero_setup(vma);
> @@ -2797,11 +2803,15 @@ static int call_mmap_prepare(struct mmap_state *map,
> int err;
>
> /* Invoke the hook. */
> - err = vfs_mmap_prepare(map->file, desc);
> + err = vfs_mmap_prepare(map->vm_file, desc);
> if (err)
> return err;
>
> - /* It's invalid for mmap_preprare hooks to clear vm_ops. */
> + /* Update first so file refcount tracked correctly. */
> + if (desc->vm_file != map->vm_file)
> + map->vm_file = desc->vm_file;
> +
> + /* It's invalid for mmap_prepare hooks to clear vm_ops. */
The less rare of prepare ;)
> if (!desc->vm_ops)
> return -EINVAL;
>
> @@ -2811,10 +2821,6 @@ static int call_mmap_prepare(struct mmap_state *map,
>
> /* Update fields permitted to be changed. */
> map->pgoff = desc->pgoff;
> - if (desc->vm_file != map->file) {
> - map->file_doesnt_need_get = true;
> - map->file = desc->vm_file;
> - }
> map->vma_flags = desc->vma_flags;
> map->page_prot = desc->page_prot;
> /* User-defined fields. */
> @@ -2826,7 +2832,7 @@ static int call_mmap_prepare(struct mmap_state *map,
> * anonymous mappings. Rather than allowing these mappings to be odd
> * outliers, simply make them truly anonymous.
> */
> - if (map_is_private(map) && file_is_dev_zero(map->file))
> + if (map_is_private(map) && file_is_dev_zero(map->vm_file))
> map_set_anon(map);
>
> return 0;
> @@ -2845,7 +2851,7 @@ static void set_vma_user_defined_fields(struct vm_area_struct *vma,
> */
> static bool can_set_ksm_flags_early(struct mmap_state *map)
> {
> - struct file *file = map->file;
> + struct file *file = map->vm_file;
>
> /* Anonymous mappings have no driver which can change them. */
> if (!file)
> @@ -2868,6 +2874,20 @@ static bool can_set_ksm_flags_early(struct mmap_state *map)
> return false;
> }
>
> +static void put_map(struct mmap_state *map)
I like the put_map_file() instead, like Suren suggested.. but maybe
put_map_vm_file(), especially since it could be read as put to the file
pointer instead of vm_file.
> +{
> + /*
> + * An error occurred or the VMA was merged.
> + *
> + * If the file was changed by the driver (which is required to increment
> + * the replacement file's reference count), drop its reference count.
> + *
> + * On error, the caller always drops the original file regardless.
> + */
> + if (map->vm_file && !map_same_file(map))
> + fput(map->vm_file);
> +}
> +
> static unsigned long __mmap_region(struct file *file, unsigned long addr,
> unsigned long len, vma_flags_t vma_flags,
> unsigned long pgoff, struct list_head *uf)
> @@ -2922,7 +2942,10 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
>
> __mmap_complete(&map, vma);
>
> - if (have_mmap_prepare && allocated_new) {
> + if (!allocated_new) {
> + /* Merged, so need to drop refcount. */
> + put_map(&map);
> + } else if (have_mmap_prepare) {
> error = mmap_action_complete(vma, &desc.action,
> /*is_compat=*/false);
> if (error)
> @@ -2936,13 +2959,7 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
> if (map.charged)
> vm_unacct_memory(map.charged);
> abort_munmap:
> - /*
> - * This indicates that .mmap_prepare has set a new file, differing from
> - * desc->vm_file. But since we're aborting the operation, only the
> - * original file will be cleaned up. Ensure we clean up both.
> - */
> - if (map.file_doesnt_need_get)
> - fput(map.file);
> + put_map(&map);
> vms_abort_munmap_vmas(&map.vms, &map.mas_detach);
> return error;
> }
> diff --git a/mm/vma.h b/mm/vma.h
> index e97bd2dfa786..f15faa83f3d6 100644
> --- a/mm/vma.h
> +++ b/mm/vma.h
> @@ -394,8 +394,10 @@ static inline void compat_set_vma_from_desc(struct vm_area_struct *vma,
>
> /* Mutable fields. Populated with initial state. */
> vma_set_pgoff(vma, desc->pgoff);
> - if (desc->vm_file != vma->vm_file)
> - vma_set_file(vma, desc->vm_file);
> + if (desc->vm_file != vma->vm_file) {
> + fput(vma->vm_file);
> + vma->vm_file = desc->vm_file;
This isn't going to be racy somehow, right?
> + }
> vma->flags = desc->vma_flags;
> vma->vm_page_prot = desc->page_prot;
>
>
> --
> 2.55.0
>
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