On Thu, 3 Mar 2022, David Hildenbrand wrote: > 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()) Hah, I'm showing my age there, or the patch's age: it's been rebranded frequently since then, with linux-next calling it free_zone_device_page(), as you kindly point out. How long before it's free_zone_device_folio()? > > 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/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()? I'm sure you could tell me a lot more about ZONE_DEVICE pages than I could ever tell you. But, if they don't ever reach the main page freer, then they're in the same category as other pages not freed until reboot: any clearing of left-behind PG_waiters will be done by the next waker, not by reaching free_pages_prepare(). Does that really require special mention of ZONE_DEVICE pages here? Would I do better just to remove the comment on PAGE_FLAGS_CHECK_AT_PREP being one of the clearers? (I had to do a bit of research before answering: temporarily confused about the role of PG_waiters, I worried that removing copy-and-pasted __ClearPageWaiters from free_zone_device_page() might risk gradually clogging up the hash queues with spuriously waited pages; no, nonsense, it's just a matter of how efficient the next folio_unlock() will be.) Hugh
