On 03.03.22 02:56, Hugh Dickins wrote: > The PG_waiters bit is not included in PAGE_FLAGS_CHECK_AT_FREE, and > vmscan.c's free_unref_page_list() callers rely on that not to generate > bad_page() alerts. So __page_cache_release() and release_pages() (and > the presumably copy-and-pasted put_zone_device_private_or_public_page()) > are redundant and misleading to make a special point of clearing it (as > the "__" implies, it could only safely be used on the freeing path). > > Delete __ClearPageWaiters(). Remark on this in one of the "possible" > comments in wake_up_page_bit(), and delete the superfluous comments. > > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> > --- > We've used this since 2018, and I see Yu Zhao posted similar in 2020: > https://lore.kernel.org/linux-mm/20200818184704.3625199-3-yuzhao@xxxxxxxxxx/ > I couldn't join in at that time, but think its reception was over-cautious. > > include/linux/page-flags.h | 2 +- > mm/filemap.c | 22 +++++++--------------- > mm/memremap.c | 2 -- > mm/swap.c | 4 ---- > 4 files changed, 8 insertions(+), 22 deletions(-) > > --- a/include/linux/page-flags.h > +++ b/include/linux/page-flags.h > @@ -481,7 +481,7 @@ static unsigned long *folio_flags(struct folio *folio, unsigned n) > TESTSETFLAG_FALSE(uname, lname) TESTCLEARFLAG_FALSE(uname, lname) > > __PAGEFLAG(Locked, locked, PF_NO_TAIL) > -PAGEFLAG(Waiters, waiters, PF_ONLY_HEAD) __CLEARPAGEFLAG(Waiters, waiters, PF_ONLY_HEAD) > +PAGEFLAG(Waiters, waiters, PF_ONLY_HEAD) > PAGEFLAG(Error, error, PF_NO_TAIL) TESTCLEARFLAG(Error, error, PF_NO_TAIL) > PAGEFLAG(Referenced, referenced, PF_HEAD) > TESTCLEARFLAG(Referenced, referenced, PF_HEAD) > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -1179,24 +1179,16 @@ static void folio_wake_bit(struct folio *folio, int bit_nr) > } > > /* > - * It is possible for other pages to have collided on the waitqueue > - * hash, so in that case check for a page match. That prevents a long- > - * term waiter > + * It's possible to miss clearing waiters here, when we woke our page > + * waiters, but the hashed waitqueue has waiters for other pages on it. > * > - * It is still possible to miss a case here, when we woke page waiters > - * and removed them from the waitqueue, but there are still other > - * page waiters. > + * That's okay, it's a rare case. The next waker will clear it. Or, > + * it might be left set until the page is freed: when it's masked off > + * with others in PAGE_FLAGS_CHECK_AT_PREP, by free_pages_prepare(). > */ Does that also apply to ZONE_DEVICE pages via free_zone_device_page()? -- Thanks, David / dhildenb
