Re: [PATCH v3 1/9] mm: fix eager write-bit restore for write-notified shared mappings

[sashiko-bot@kernel.org]

> change_protection() applies vma->vm_page_prot, which lacks the write bit
> for CoW and write-notified mappings. Callers set
> MM_CP_TRY_CHANGE_WRITABLE when vma_wants_manual_pte_write_upgrade()
> returns true, so that entries that can be written without a fault, such
> as dirty PTEs in a shared mapping, are made writable right away.
> 
> For shared mappings, that function passes vma->vm_page_prot to
> vma_wants_writenotify(). Once writenotify is enabled, this protection
> lacks the write bit, while the vma_pgprot_modify() result it is compared
> against adds it back. The two never match, so vma_wants_writenotify()
> returns false before checking for soft-dirty, uffd-wp and writeback.
> 
> As a result, write-notified shmem mappings never get
> MM_CP_TRY_CHANGE_WRITABLE. For example, commit 931298e103c2
> ("mm/userfaultfd: rely on vma->vm_page_prot in uffd_wp_range()") made
> uffd-wp unprotect set the flag so that dirty shmem PTEs become writable
> again, but this never takes effect. Unprotecting N dirty shmem pages
> still leads to N faults that only set the write bit.
> 
> This dates back to the commit in the Fixes tag, which moved the
> equivalent check in mprotect_fixup() before vma_set_page_prot().
> 
> Fix this by passing the protection recomputed from the VMA flags, as
> vma_set_page_prot() does.
> 
> Note: this only affects performance, as the next write fault restores
> the write bit anyway.
> 
> Fixes: 64e455079e1b ("mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared")
> Signed-off-by: Carlos López <clopez@suse.de>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261112142039.907090-2-clopez@suse.de?part=1