mm/shmem: unconditionally set pte dirty in mfill_atomic_install_pte

Patch series "mm: A few cleanup patches around zap, shmem and uffd", v4.

IMHO all of them are very nice cleanups to existing code already,
they're all small and self-contained.  They'll be needed by uffd-wp
coming series.

This patch (of 4):

It was conditionally done previously, as there's one shmem special case
that we use SetPageDirty() instead.  However that's not necessary and it
should be easier and cleaner to do it unconditionally in
mfill_atomic_install_pte().

The most recent discussion about this is here, where Hugh explained the
history of SetPageDirty() and why it's possible that it's not required
at all:

https://lore.kernel.org/lkml/alpine.LSU.2.11.2104121657050[email protected]/

Currently mfill_atomic_install_pte() has three callers:

        1. shmem_mfill_atomic_pte
        2. mcopy_atomic_pte
        3. mcontinue_atomic_pte

After the change: case (1) should have its SetPageDirty replaced by the
dirty bit on pte (so we unify them together, finally), case (2) should
have no functional change at all as it has page_in_cache==false, case
(3) may add a dirty bit to the pte.  However since case (3) is
UFFDIO_CONTINUE for shmem, it's merely 100% sure the page is dirty after
all because UFFDIO_CONTINUE normally requires another process to modify
the page cache and kick the faulted thread, so should not make a real
difference either.

This should make it much easier to follow on which case will set dirty
for uffd, as we'll simply set it all now for all uffd related ioctls.
Meanwhile, no special handling of SetPageDirty() if there's no need.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Peter Xu <[email protected]>
Reviewed-by: Axel Rasmussen <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Liam Howlett <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: Jerome Glisse <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/mm/shmem.c b/mm/shmem.c
index 7cd976b588ff1c5f06d7abde610655c449ef4db7..df2f0288b9caa57af014c8d382d95a64e8c1d5d2 100644 (file)
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2423,7 +2423,6 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
        shmem_recalc_inode(inode);
        spin_unlock_irq(&info->lock);
 
-       SetPageDirty(page);
        unlock_page(page);
        return 0;
 out_delete_from_cache:

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 7a9008415534312b29ab5ad8aaa5e5ad03adc52b..caf6dfff2a60f7d582a8e00da67489919d5d86e8 100644 (file)
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -69,10 +69,9 @@ int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
        pgoff_t offset, max_off;
 
        _dst_pte = mk_pte(page, dst_vma->vm_page_prot);
+       _dst_pte = pte_mkdirty(_dst_pte);
        if (page_in_cache && !vm_shared)
                writable = false;
-       if (writable || !page_in_cache)
-               _dst_pte = pte_mkdirty(_dst_pte);
        if (writable) {
                if (wp_copy)
                        _dst_pte = pte_mkuffd_wp(_dst_pte);