| Message ID | 20240322210455.3738-1-soma.nakata01@gmail.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm/filemap: set folio->mapping to NULL before xas_store() | expand |
On Sat, 23 Mar 2024 06:04:54 +0900 Soma Nakata <soma.nakata01@gmail.com> 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 Tue, Mar 26, 2024 at 02:05:33PM -0700, Andrew Morton wrote: > On Sat, 23 Mar 2024 06:04:54 +0900 Soma Nakata <soma.nakata01@gmail.com> 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.
On Wed, Mar 27, 2024 at 6:05 AM Andrew Morton <akpm@linux-foundation.org> wrote: > > On Sat, 23 Mar 2024 06:04:54 +0900 Soma Nakata <soma.nakata01@gmail.com> 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,
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; }
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 <soma.nakata01@gmail.com> --- mm/filemap.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)