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([2620:10d:c092:500::7:fe7a]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-a6c8070c050sm227963566b.144.2024.06.07.03.24.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 07 Jun 2024 03:24:14 -0700 (PDT) Message-ID: Date: Fri, 7 Jun 2024 11:24:13 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] mm: clear pte for folios that are zero filled To: David Hildenbrand , akpm@linux-foundation.org, shakeel.butt@linux.dev, yosryahmed@google.com, willy@infradead.org Cc: hannes@cmpxchg.org, nphamcs@gmail.com, chengming.zhou@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <20240604105950.1134192-1-usamaarif642@gmail.com> <20240604105950.1134192-2-usamaarif642@gmail.com> <6b1485b6-c2a1-45b8-8afe-7b211689070b@redhat.com> Content-Language: en-US From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 04/06/2024 13:43, David Hildenbrand wrote: > On 04.06.24 14:30, David Hildenbrand wrote: >> On 04.06.24 12:58, Usama Arif wrote: >>> Approximately 10-20% of pages to be swapped out are zero pages [1]. >>> Rather than reading/writing these pages to flash resulting >>> in increased I/O and flash wear, the pte can be cleared for those >>> addresses at unmap time while shrinking folio list. When this >>> causes a page fault, do_pte_missing will take care of this page. >>> With this patch, NVMe writes in Meta server fleet decreased >>> by almost 10% with conventional swap setup (zswap disabled). >>> >>> [1] >>> https://lore.kernel.org/all/20171018104832epcms5p1b2232e2236258de3d03d1344dde9fce0@epcms5p1/ >>> >>> Signed-off-by: Usama Arif >>> --- >>>    include/linux/rmap.h |   1 + >>>    mm/rmap.c            | 163 >>> ++++++++++++++++++++++--------------------- >>>    mm/vmscan.c          |  89 ++++++++++++++++------- >>>    3 files changed, 150 insertions(+), 103 deletions(-) >>> >>> diff --git a/include/linux/rmap.h b/include/linux/rmap.h >>> index bb53e5920b88..b36db1e886e4 100644 >>> --- a/include/linux/rmap.h >>> +++ b/include/linux/rmap.h >>> @@ -100,6 +100,7 @@ enum ttu_flags { >>>                         * do a final flush if necessary */ >>>        TTU_RMAP_LOCKED        = 0x80,    /* do not grab rmap lock: >>>                         * caller holds it */ >>> +    TTU_ZERO_FOLIO        = 0x100,/* zero folio */ >>>    }; >>>       #ifdef CONFIG_MMU >>> diff --git a/mm/rmap.c b/mm/rmap.c >>> index 52357d79917c..d98f70876327 100644 >>> --- a/mm/rmap.c >>> +++ b/mm/rmap.c >>> @@ -1819,96 +1819,101 @@ static bool try_to_unmap_one(struct folio >>> *folio, struct vm_area_struct *vma, >>>                 */ >>>                dec_mm_counter(mm, mm_counter(folio)); >>>            } else if (folio_test_anon(folio)) { >>> -            swp_entry_t entry = page_swap_entry(subpage); >>> -            pte_t swp_pte; >>> -            /* >>> -             * Store the swap location in the pte. >>> -             * See handle_pte_fault() ... >>> -             */ >>> -            if (unlikely(folio_test_swapbacked(folio) != >>> -                    folio_test_swapcache(folio))) { >>> +            if (flags & TTU_ZERO_FOLIO) { >>> +                pte_clear(mm, address, pvmw.pte); >>> +                dec_mm_counter(mm, MM_ANONPAGES); >> >> Is there an easy way to reduce the code churn and highlight the added >> code? >> >> Like >> >> } else if (folio_test_anon(folio) && (flags & TTU_ZERO_FOLIO)) { >> >> } else if (folio_test_anon(folio)) { >> >> >> >> Also to concerns that I want to spell out: >> >> (a) what stops the page from getting modified in the meantime? The CPU >>       can write it until the TLB was flushed. >> Thanks for pointing this out David and Shakeel. This is a big issue in this v2, and as Shakeel pointed out in [1] we need to do a second rmap walk. Looking at how ksm deals with this in try_to_merge_one_page which calls write_protect_page for each vma (i.e. basically an rmap walk), this would be much more CPU expensive and complicated compared to v1 [2], where the swap subsystem can handle all complexities. I will go back to my v1 solution for the next revision as its much more simpler and the memory usage is very low (0.003%) as pointed out by Johannes [3] which would likely go away with the memory savings of not having a zswap_entry for zero filled pages, and the solution being a lot simpler than what a valid v2 approach would look like. [1] https://lore.kernel.org/all/nes73bwc5p6yhwt5tw3upxcqrn5kenn6lvqb6exrf4yppmz6jx@ywhuevpkxlvh/ [2] https://lore.kernel.org/all/20240530102126.357438-1-usamaarif642@gmail.com/ [3] https://lore.kernel.org/all/20240530122715.GB1222079@cmpxchg.org/ >> (b) do you properly handle if the page is pinned (or just got pinned) >>       and we must not discard it? > > Oh, and I forgot, are you handling userfaultd as expected? IIRC there > are some really nasty side-effects with userfaultfd even when > userfaultfd is currently not registered for a VMA [1]. > > [1] > https://lore.kernel.org/linux-mm/3a4b1027-df6e-31b8-b0de-ff202828228d@redhat.com/ > > What should work is replacing all-zero anonymous pages by the shared > zeropage iff the anonymous page is not pinned and we synchronize > against GUP fast. Well, and we handle possible concurrent writes > accordingly. > > KSM does essentially that when told to de-duplicate the shared > zeropage, and I was thinking a while ago if we would want a > zeropage-only KSM version that doesn't need stable tress and all that, > but only deduplicates zero-filled pages into the shared zeropage in a > safe way. > Thanks for the pointer to KSM code.