Re: [PATCH v3 4/9] mm: keep writable entries writable on protection changes

[sashiko-bot@kernel.org]

> change_protection() applies vma->vm_page_prot, which lacks the write bit
> for CoW and write-notified mappings. MM_CP_TRY_CHANGE_WRITABLE makes
> entries writable again if a write fault would only set the write bit.
> In shared mappings, it relies on the dirty bit for this, as a clean
> entry might still need a write fault to notify the FS.
> 
> However, this misses clean entries that were already writable. For
> example, a uffd-wp unprotect makes shmem PTEs read-only if they were
> mapped by a read fault before uffd-wp was registered, so the next write
> to each of them faults.
> 
> Entries that were writable before the change can stay writable, as
> anything that relies on write faults write-protects the entry first, and
> MM_CP_TRY_CHANGE_WRITABLE is only set if the VMA stays writable. So
> check the old entry before the shared and private rules.
> 
> Note: writable anonymous entries are always exclusive and thus already
> kept writable, so in private mappings this only affects PFNs that
> drivers map writable into them.
> 
> Signed-off-by: Carlos López <clopez@suse.de>

Sashiko has reviewed this patch and found no issues.

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