The patch titled Subject: mm: fix old/young bit handling in the faulting path has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-fix-old-young-bit-handling-in-the-faulting-path.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via the mm-everything branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there every 2-3 working days

------------------------------------------------------ From: Ram Tummala rtummala@nvidia.com Subject: mm: fix old/young bit handling in the faulting path Date: Tue, 9 Jul 2024 18:45:39 -0700

Commit 3bd786f76de2 ("mm: convert do_set_pte() to set_pte_range()") replaced do_set_pte() with set_pte_range() and that introduced a regression in the following faulting path of non-anonymous vmas which caused the PTE for the faulting address to be marked as old instead of young.

handle_pte_fault() do_pte_missing() do_fault() do_read_fault() || do_cow_fault() || do_shared_fault() finish_fault() set_pte_range()

The polarity of prefault calculation is incorrect. This leads to prefault being incorrectly set for the faulting address. The following check will incorrectly mark the PTE old rather than young. On some architectures this will cause a double fault to mark it young when the access is retried.

if (prefault && arch_wants_old_prefaulted_pte()) entry = pte_mkold(entry);

On a subsequent fault on the same address, the faulting path will see a non NULL vmf->pte and instead of reaching the do_pte_missing() path, PTE will then be correctly marked young in handle_pte_fault() itself.

Due to this bug, performance degradation in the fault handling path will be observed due to unnecessary double faulting.

Link: https://lkml.kernel.org/r/20240710014539.746200-1-rtummala@nvidia.com Fixes: 3bd786f76de2 ("mm: convert do_set_pte() to set_pte_range()") Signed-off-by: Ram Tummala rtummala@nvidia.com Reviewed-by: Yin Fengwei fengwei.yin@intel.com Cc: Alistair Popple apopple@nvidia.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Yin Fengwei fengwei.yin@intel.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/memory.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/memory.c~mm-fix-old-young-bit-handling-in-the-faulting-path +++ a/mm/memory.c @@ -4681,7 +4681,7 @@ void set_pte_range(struct vm_fault *vmf, { struct vm_area_struct *vma = vmf->vma; bool write = vmf->flags & FAULT_FLAG_WRITE; - bool prefault = in_range(vmf->address, addr, nr * PAGE_SIZE); + bool prefault = !in_range(vmf->address, addr, nr * PAGE_SIZE); pte_t entry;

flush_icache_pages(vma, page, nr); _

Patches currently in -mm which might be from rtummala@nvidia.com are

mm-fix-old-young-bit-handling-in-the-faulting-path.patch