On 03.03.22 23:28, Hugh Dickins wrote: > 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? In this context we can consider ZONE_DEVICE pages just like any other pages that, although getting freed, are not returned to the buddy, but instead are returned to another pool. So PAGE_FLAGS_CHECK_AT_PREP won't apply and free_pages_prepare() won't apply. Another example would be hugetlb pages, that are returned to the hugetlb pool, but not back to the buddy unless the huge page pool is shrunk. So I feel like the underlying principle here is: we don't *care* if PG_waiter is cleared when a page gets freed, because it will simply get cleared by the next waker if it sticks around. Then, I agree, we can just drop the comment regarding PAGE_FLAGS_CHECK_AT_PREP and instead have something like " That's okay, it's a rare case and the next waker will just clear it. Note that, depending on the page pool (buddy, ZONE_DEVICE, hugetlb), we might clear the flag while freeing the page, however, this is not required for correctness. " -- Thanks, David / dhildenb
