Functions such as __filemap_get_folio() check the truncation of
folios based on the mapping field. Therefore setting this field to NULL
earlier prevents unnecessary operations on already removed folios.
Signed-off-by: Soma Nakata <[email protected]>
---
mm/filemap.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/mm/filemap.c b/mm/filemap.c
index 2723104cc06a..79bac7c00084 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -139,11 +139,12 @@ static void page_cache_delete(struct address_space *mapping,
VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
+ folio->mapping = NULL;
+ /* Leave page->index set: truncation lookup relies upon it */
+
xas_store(&xas, shadow);
xas_init_marks(&xas);
- folio->mapping = NULL;
- /* Leave page->index set: truncation lookup relies upon it */
mapping->nrpages -= nr;
}
--
2.25.1
On Sat, 23 Mar 2024 06:04:54 +0900 Soma Nakata <[email protected]> wrote:
> Functions such as __filemap_get_folio() check the truncation of
> folios based on the mapping field. Therefore setting this field to NULL
> earlier prevents unnecessary operations on already removed folios.
>
> ...
>
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -139,11 +139,12 @@ static void page_cache_delete(struct address_space *mapping,
>
> VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
>
> + folio->mapping = NULL;
> + /* Leave page->index set: truncation lookup relies upon it */
> +
> xas_store(&xas, shadow);
> xas_init_marks(&xas);
>
> - folio->mapping = NULL;
> - /* Leave page->index set: truncation lookup relies upon it */
> mapping->nrpages -= nr;
> }
Seems at least harmless, but I wonder if it can really make any
difference. Don't readers of folio->mapping lock the folio first?
On Wed, Mar 27, 2024 at 6:05 AM Andrew Morton <[email protected]> wrote:
>
> On Sat, 23 Mar 2024 06:04:54 +0900 Soma Nakata <[email protected]> wrote:
>
> > Functions such as __filemap_get_folio() check the truncation of
> > folios based on the mapping field. Therefore setting this field to NULL
> > earlier prevents unnecessary operations on already removed folios.
> >
> > ...
> >
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -139,11 +139,12 @@ static void page_cache_delete(struct address_space *mapping,
> >
> > VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
> >
> > + folio->mapping = NULL;
> > + /* Leave page->index set: truncation lookup relies upon it */
> > +
> > xas_store(&xas, shadow);
> > xas_init_marks(&xas);
> >
> > - folio->mapping = NULL;
> > - /* Leave page->index set: truncation lookup relies upon it */
> > mapping->nrpages -= nr;
> > }
>
> Seems at least harmless, but I wonder if it can really make any
> difference. Don't readers of folio->mapping lock the folio first?
Yes, the reader locks the folio.
Only __filemap_remove_folio() calls page_cache_delete(),
and it says the caller has to lock the folio or make sure
that usage is safe. In the latter case, this patch improves
efficiency a little bit.
However, I found that there is not any latter case actually,
so discard it or apply, also to make the order of operations in
page_cache_delete() and page_cache_delete_batch() the same
for a cleanup.
Thanks,
On Tue, Mar 26, 2024 at 02:05:33PM -0700, Andrew Morton wrote:
> On Sat, 23 Mar 2024 06:04:54 +0900 Soma Nakata <[email protected]> wrote:
> > Functions such as __filemap_get_folio() check the truncation of
> > folios based on the mapping field. Therefore setting this field to NULL
> > earlier prevents unnecessary operations on already removed folios.
> >
> > ...
> >
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -139,11 +139,12 @@ static void page_cache_delete(struct address_space *mapping,
> >
> > VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
> >
> > + folio->mapping = NULL;
> > + /* Leave page->index set: truncation lookup relies upon it */
> > +
> > xas_store(&xas, shadow);
> > xas_init_marks(&xas);
> >
> > - folio->mapping = NULL;
> > - /* Leave page->index set: truncation lookup relies upon it */
> > mapping->nrpages -= nr;
> > }
>
> Seems at least harmless, but I wonder if it can really make any
> difference. Don't readers of folio->mapping lock the folio first?
I can't think of anywhere that doesn't ... most of the places that check
folio->mapping have "goto unlock" as the very next line. I don't think
this patch accomplishes anything.