[PATCH 1/2] mm: Allow architectures to request 'old' entries when prefaulting

[Hugh Dickins]

On Sun, 27 Dec 2020, Damian Tometzki wrote:
> On Sun, 27. Dec 11:38, Linus Torvalds wrote:
> > On Sat, Dec 26, 2020 at 6:38 PM Hugh Dickins <hughd at google.com> wrote:
> > >
> > > This patch (like its antecedents) moves the pte_unmap_unlock() from
> > > after do_fault_around()'s "check if the page fault is solved" into
> > > filemap_map_pages() itself (which apparently does not NULLify vmf->pte
> > > after unmapping it, which is poor, but good for revealing this issue).
> > > That looks cleaner, but of course there was a very good reason for its
> > > original positioning.
> > 
> > Good catch.
> > 
> > > Maybe you want to change the ->map_pages prototype, to pass down the
> > > requested address too, so that it can report whether the requested
> > > address was resolved or not.  Or it could be left to __do_fault(),
> > > or even to a repeated fault; but those would be less efficient.
> > 
> > Let's keep the old really odd "let's unlock in the caller" for now,
> > and minimize the changes.
> > 
> > Adding a big big comment at the end of filemap_map_pages() to note the
> > odd delayed page table unlocking.
> > 
> > Here's an updated patch that combines Kirill's original patch, his
> > additional incremental patch, and the fix for the pte lock oddity into
> > one thing.
> > 
> > Does this finally pass your testing?

Yes, this one passes my testing on x86_64 and on i386.  But...

> > 
> >                Linus
> Hello together,
> 
> when i try to build this patch, i got the following error:
> 
>  CC      arch/x86/kernel/cpu/mce/threshold.o
> mm/memory.c:3716:19: error: static declaration of ‘do_set_pmd’ follows non-static declaration
>  static vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
>                    ^~~~~~~~~~
> In file included from mm/memory.c:43:
> ./include/linux/mm.h:984:12: note: previous declaration of ‘do_set_pmd’ was here
>  vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page);
>             ^~~~~~~~~~
> make[3]: *** [scripts/Makefile.build:279: mm/memory.o] Error 1
> make[2]: *** [Makefile:1805: mm] Error 2
> make[2]: *** Waiting for unfinished jobs....
>   CC      arch/x86/kernel/cpu/mce/therm_throt.o

... Damian very helpfully reports that it does not build when
CONFIG_TRANSPARENT_HUGEPAGE is not set, since the "static " has
not been removed from the alternative definition of do_set_pmd().

And its BUILD_BUG() becomes invalid once it's globally available.
You don't like unnecessary BUG()s, and I don't like returning
success there: VM_FAULT_FALLBACK seems best.

--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3713,10 +3713,9 @@ out:
 	return ret;
 }
 #else
-static vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
+vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
 {
-	BUILD_BUG();
-	return 0;
+	return VM_FAULT_FALLBACK;
 }
 #endif
 

(I'm also a wee bit worried by filemap.c's +#include <asm/pgalloc.h>:
that's the kind of thing that might turn out not to work on some arch.)

Hugh