From e61abd4490684de379b4a2ef1be2dbde39ac1ced Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Wed, 14 Feb 2024 21:44:34 +0100 Subject: [PATCH] mm/mmu_gather: improve cond_resched() handling with large folios and expensive page freeing In tlb_batch_pages_flush(), we can end up freeing up to 512 pages or now up to 256 folio fragments that span more than one page, before we conditionally reschedule. It's a pain that we have to handle cond_resched() in tlb_batch_pages_flush() manually and cannot simply handle it in release_pages() -- release_pages() can be called from atomic context. Well, in a perfect world we wouldn't have to make our code more complicated at all. With page poisoning and init_on_free, we might now run into soft lockups when we free a lot of rather large folio fragments, because page freeing time then depends on the actual memory size we are freeing instead of on the number of folios that are involved. In the absolute (unlikely) worst case, on arm64 with 64k we will be able to free up to 256 folio fragments that each span 512 MiB: zeroing out 128 GiB does sound like it might take a while. But instead of ignoring this unlikely case, let's just handle it. So, let's teach tlb_batch_pages_flush() that there are some configurations where page freeing is horribly slow, and let's reschedule more frequently -- similarly like we did for now before we had large folio fragments in there. Avoid yet another loop over all encoded pages in the common case by handling that separately. Note that with page poisoning/zeroing, we might now end up freeing only a single folio fragment at a time that might exceed the old 512 pages limit: but if we cannot even free a single MAX_ORDER page on a system without running into soft lockups, something else is already completely bogus. Freeing a PMD-mapped THP would similarly cause trouble. In theory, we might even free 511 order-0 pages + a single MAX_ORDER page, effectively having to zero out 8703 pages on arm64 with 64k, translating to ~544 MiB of memory: however, if 512 MiB doesn't result in soft lockups, 544 MiB is unlikely to result in soft lockups, so we won't care about that for the time being. In the future, we might want to detect if handling cond_resched() is required at all, and just not do any of that with full preemption enabled. Link: https://lkml.kernel.org/r/20240214204435.167852-10-david@redhat.com Signed-off-by: David Hildenbrand Reviewed-by: Ryan Roberts Cc: Alexander Gordeev Cc: Aneesh Kumar K.V Cc: Arnd Bergmann Cc: Catalin Marinas Cc: Christian Borntraeger Cc: Christophe Leroy Cc: Heiko Carstens Cc: Matthew Wilcox (Oracle) Cc: Michael Ellerman Cc: Michal Hocko Cc: "Naveen N. Rao" Cc: Nicholas Piggin Cc: Peter Zijlstra (Intel) Cc: Sven Schnelle Cc: Vasily Gorbik Cc: Will Deacon Cc: Yin Fengwei Signed-off-by: Andrew Morton --- mm/mmu_gather.c | 60 ++++++++++++++++++++++++++++++++++++------------- 1 file changed, 44 insertions(+), 16 deletions(-) diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c index d175c0f1e2c8..99b3e9408aa0 100644 --- a/mm/mmu_gather.c +++ b/mm/mmu_gather.c @@ -91,18 +91,21 @@ void tlb_flush_rmaps(struct mmu_gather *tlb, struct vm_area_struct *vma) } #endif -static void tlb_batch_pages_flush(struct mmu_gather *tlb) +/* + * We might end up freeing a lot of pages. Reschedule on a regular + * basis to avoid soft lockups in configurations without full + * preemption enabled. The magic number of 512 folios seems to work. + */ +#define MAX_NR_FOLIOS_PER_FREE 512 + +static void __tlb_batch_free_encoded_pages(struct mmu_gather_batch *batch) { - struct mmu_gather_batch *batch; + struct encoded_page **pages = batch->encoded_pages; + unsigned int nr, nr_pages; - for (batch = &tlb->local; batch && batch->nr; batch = batch->next) { - struct encoded_page **pages = batch->encoded_pages; - - while (batch->nr) { - /* - * limit free batch count when PAGE_SIZE > 4K - */ - unsigned int nr = min(512U, batch->nr); + while (batch->nr) { + if (!page_poisoning_enabled_static() && !want_init_on_free()) { + nr = min(MAX_NR_FOLIOS_PER_FREE, batch->nr); /* * Make sure we cover page + nr_pages, and don't leave @@ -111,14 +114,39 @@ static void tlb_batch_pages_flush(struct mmu_gather *tlb) if (unlikely(encoded_page_flags(pages[nr - 1]) & ENCODED_PAGE_BIT_NR_PAGES_NEXT)) nr++; - - free_pages_and_swap_cache(pages, nr); - pages += nr; - batch->nr -= nr; - - cond_resched(); + } else { + /* + * With page poisoning and init_on_free, the time it + * takes to free memory grows proportionally with the + * actual memory size. Therefore, limit based on the + * actual memory size and not the number of involved + * folios. + */ + for (nr = 0, nr_pages = 0; + nr < batch->nr && nr_pages < MAX_NR_FOLIOS_PER_FREE; + nr++) { + if (unlikely(encoded_page_flags(pages[nr]) & + ENCODED_PAGE_BIT_NR_PAGES_NEXT)) + nr_pages += encoded_nr_pages(pages[++nr]); + else + nr_pages++; + } } + + free_pages_and_swap_cache(pages, nr); + pages += nr; + batch->nr -= nr; + + cond_resched(); } +} + +static void tlb_batch_pages_flush(struct mmu_gather *tlb) +{ + struct mmu_gather_batch *batch; + + for (batch = &tlb->local; batch && batch->nr; batch = batch->next) + __tlb_batch_free_encoded_pages(batch); tlb->active = &tlb->local; }