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Shutemov" , Matthew Wilcox , Vlastimil Babka , Zi Yan , Linux Kernel Mailing List , Shuang Zhai Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org () On Fri, Aug 13, 2021 at 5:24 PM Yang Shi wrote: > > On Wed, Aug 11, 2021 at 4:12 PM Yu Zhao wrote: > > > > On Wed, Aug 11, 2021 at 4:25 PM Yang Shi wrote: > > > > > > On Sun, Aug 8, 2021 at 10:49 AM Yu Zhao wrote: > > > > > > > > On Wed, Aug 4, 2021 at 6:13 PM Yang Shi wrote: > > > > > > > > > > On Fri, Jul 30, 2021 at 11:39 PM Yu Zhao wrote: > > > > > > > > > > > > If a tail page has only two references left, one inherited from the > > > > > > isolation of its head and the other from lru_add_page_tail() which we > > > > > > are about to drop, it means this tail page was concurrently zapped. > > > > > > Then we can safely free it and save page reclaim or migration the > > > > > > trouble of trying it. > > > > > > > > > > > > Signed-off-by: Yu Zhao > > > > > > Tested-by: Shuang Zhai > > > > > > --- > > > > > > include/linux/vm_event_item.h | 1 + > > > > > > mm/huge_memory.c | 28 ++++++++++++++++++++++++++++ > > > > > > mm/vmstat.c | 1 + > > > > > > 3 files changed, 30 insertions(+) > > > > > > > > > > > > diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h > > > > > > index ae0dd1948c2b..829eeac84094 100644 > > > > > > --- a/include/linux/vm_event_item.h > > > > > > +++ b/include/linux/vm_event_item.h > > > > > > @@ -99,6 +99,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT, > > > > > > #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD > > > > > > THP_SPLIT_PUD, > > > > > > #endif > > > > > > + THP_SPLIT_FREE, > > > > > > THP_ZERO_PAGE_ALLOC, > > > > > > THP_ZERO_PAGE_ALLOC_FAILED, > > > > > > THP_SWPOUT, > > > > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > > > > > > index d8b655856e79..5120478bca41 100644 > > > > > > --- a/mm/huge_memory.c > > > > > > +++ b/mm/huge_memory.c > > > > > > @@ -2432,6 +2432,8 @@ static void __split_huge_page(struct page *page, struct list_head *list, > > > > > > struct address_space *swap_cache = NULL; > > > > > > unsigned long offset = 0; > > > > > > unsigned int nr = thp_nr_pages(head); > > > > > > + LIST_HEAD(pages_to_free); > > > > > > + int nr_pages_to_free = 0; > > > > > > int i; > > > > > > > > > > > > VM_BUG_ON_PAGE(list && PageLRU(head), head); > > > > > > @@ -2506,6 +2508,25 @@ static void __split_huge_page(struct page *page, struct list_head *list, > > > > > > continue; > > > > > > unlock_page(subpage); > > > > > > > > > > > > + /* > > > > > > + * If a tail page has only two references left, one inherited > > > > > > + * from the isolation of its head and the other from > > > > > > + * lru_add_page_tail() which we are about to drop, it means this > > > > > > + * tail page was concurrently zapped. Then we can safely free it > > > > > > + * and save page reclaim or migration the trouble of trying it. > > > > > > + */ > > > > > > + if (list && page_ref_freeze(subpage, 2)) { > > > > > > + VM_BUG_ON_PAGE(PageLRU(subpage), subpage); > > > > > > + VM_BUG_ON_PAGE(PageCompound(subpage), subpage); > > > > > > + VM_BUG_ON_PAGE(page_mapped(subpage), subpage); > > > > > > + > > > > > > + ClearPageActive(subpage); > > > > > > + ClearPageUnevictable(subpage); > > > > > > + list_move(&subpage->lru, &pages_to_free); > > > > > > + nr_pages_to_free++; > > > > > > + continue; > > > > > > + } > > > > > > > > > > Yes, such page could be freed instead of swapping out. But I'm > > > > > wondering if we could have some simpler implementation. Since such > > > > > pages will be re-added to page list, so we should be able to check > > > > > their refcount in shrink_page_list(). If the refcount is 1, the > > > > > refcount inc'ed by lru_add_page_tail() has been put by later > > > > > put_page(), we know it is freed under us since the only refcount comes > > > > > from isolation, we could just jump to "keep" (the label in > > > > > shrink_page_list()), then such page will be freed later by > > > > > shrink_inactive_list(). > > > > > > > > > > For MADV_PAGEOUT, I think we could add some logic to handle such page > > > > > after shrink_page_list(), just like what shrink_inactive_list() does. > > > > > > > > > > Migration already handles refcount == 1 page, so should not need any change. > > > > > > > > > > Is this idea feasible? > > > > > > > > Yes, but then we would have to loop over the tail pages twice, here > > > > and in shrink_page_list(), right? > > > > > > I don't quite get what you mean "loop over the tail pages twice". Once > > > THP is isolated then get split, all the tail pages will be put on the > > > list (local list for isolated pages), then the reclaimer would deal > > > with the head page, then continue to iterate the list to deal with > > > tail pages. Your patch could free the tail pages earlier. But it > > > should not make too much difference to free the tail pages a little > > > bit later IMHO. > > > > We are in a (the first) loop here. If we free the tail pages later, > > then we will need to loop over them again (the second). > > > > IOW, > > 1) __split_huge_page(): for each of the 511 tail pages (first loop). > > 2) shrink_page_list(): for each of the 511 tail pages (second loop). > > > > > > In addition, if we try to freeze the refcount of a page in > > > > shrink_page_list(), we couldn't be certain whether this page used to > > > > be a tail page. So we would have to test every page. If a page wasn't > > > > a tail page, it's unlikely for its refcount to drop unless there is a > > > > race. But this patch isn't really intended to optimize such a race. > > > > It's mainly for the next, i.e., we know there is a good chance to drop > > > > tail pages (~10% on our systems). Sounds reasonable? Thanks. > > > > > > I'm not sure what is the main source of the partial mapped THPs from > > > your fleets. But if most of them are generated by MADV_DONTNEED (this > > > is used by some userspace memory allocator libs), they should be on > > > deferred split list too. Currently deferred split shrinker just > > > shrinks those THPs (simply split them and free unmapped sub pages) > > > proportionally, we definitely could shrink them more aggressively, for > > > example, by setting shrinker->seeks to 0. I'm wondering if this will > > > achieve a similar effect or not. > > > > Not partially mapped but internal fragmentation. > > > > IOW, some of the 4KB pages within a THP were never written into, which > > can be common depending on the implementations of userspace memory > > allocators. > > OK, this is actually what the patch #3 does. The patch #3 just doesn't > remap the "all zero" page when splitting the THP IIUC. But the page > has refcount from isolation so it can't be simply freed by put_page(). > > Actually this makes me think my suggestion is better. It doesn't make > too much sense to me to have page free logic (manipulate flags, > uncharge memcg, etc) in THP split. > > There have been a couple of places to handle such cases: > - deferred split shrinker: the unmapped subpage is just freed by > put_page() since there is no extra refcount > - migration: check page refcount then free the refcount == 1 one > > Here you add the third case in the page reclaim path, so why not just > let the page reclaim handle all the work for freeing page? As I have explained previously: 1) We would have to loop over tail pages twice. Not much overhead but unnecessary. 2) We would have to try to freeze the refcount on _every_ page in shrink_page_list() -- shrink_page_list() takes all pages, not just relevant ones (previously being tail). Attempting to freeze refcount on an irrelevant page will likely fail. Again, not a significant overhead but better to avoid. I'm not against your idea. But I'd like to hear some clarifications about the points above. That is whether you think it's still a good idea to do what you suggested after taking these into account. > > > I really don't have any objection to free such pages, but just > > > wondering if we could have something simpler or not. > > > > Thanks. > > > > > > > > + > > > > > > /* > > > > > > * Subpages may be freed if there wasn't any mapping > > > > > > * like if add_to_swap() is running on a lru page that > > > > > > @@ -2515,6 +2536,13 @@ static void __split_huge_page(struct page *page, struct list_head *list, > > > > > > */ > > > > > > put_page(subpage); > > > > > > } > > > > > > + > > > > > > + if (!nr_pages_to_free) > > > > > > + return; > > > > > > + > > > > > > + mem_cgroup_uncharge_list(&pages_to_free); > > > > > > + free_unref_page_list(&pages_to_free); > > > > > > + count_vm_events(THP_SPLIT_FREE, nr_pages_to_free); > > > > > > } > > > > > > > > > > > > int total_mapcount(struct page *page) > > > > > > diff --git a/mm/vmstat.c b/mm/vmstat.c > > > > > > index b0534e068166..f486e5d98d96 100644 > > > > > > --- a/mm/vmstat.c > > > > > > +++ b/mm/vmstat.c > > > > > > @@ -1300,6 +1300,7 @@ const char * const vmstat_text[] = { > > > > > > #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD > > > > > > "thp_split_pud", > > > > > > #endif > > > > > > + "thp_split_free", > > > > > > "thp_zero_page_alloc", > > > > > > "thp_zero_page_alloc_failed", > > > > > > "thp_swpout", > > > > > > -- > > > > > > 2.32.0.554.ge1b32706d8-goog > > > > > >