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b=cvhC5AvKs5YOK2xPFn3qFwnu5neSKjvB8xwF0vZIK81AXnH6oC/TFLnji2N1e/gsW sX5fgw0bYSCwtQiE62KKDCZefdLnA7jfM3acnRf2/nAQElN3lLZh8ry7HgfE+JRqdl plLhfPghcTwpxbjfaI6mTc1uDc9BmTOapgt3NVhNfYVKKbtzTACcte2NnCwpfYM/OA T4NaoSaDf4qIhE5ZOcuK6WSdEIgMY5MwHGTKA0lkQBoh5ekBVXW5aQoSkfZB/Hdjnd 5oERSoCmsgQx4AETmxWiSKCtN0qighjsZXzZr757naF4Vg4FmucY6uwbkADyjR2QWD P5iyZ/S4eKyug== Message-ID: Date: Fri, 26 Jun 2026 18:25:48 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/4] mm: avoid unnecessary lru drain for wp_can_reuse_anon_folio() To: Barry Song Cc: akpm@linux-foundation.org, linux-mm@kvack.org, baoquan.he@linux.dev, chrisl@kernel.org, jp.kobryn@linux.dev, kasong@tencent.com, liam@infradead.org, linux-kernel@vger.kernel.org, ljs@kernel.org, mhocko@suse.com, nphamcs@gmail.com, rppt@kernel.org, shakeel.butt@linux.dev, shikemeng@huaweicloud.com, surenb@google.com, usama.arif@linux.dev, vbabka@kernel.org, youngjun.park@lge.com References: <20260623231635.43086-1-baohua@kernel.org> <20260623231635.43086-2-baohua@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: DA9108000B X-Rspam-User: X-Stat-Signature: k5yz5dn8yiysxknzu75dcibm6wee8jd9 X-HE-Tag: 1782491155-503908 X-HE-Meta: U2FsdGVkX1+xRWLbSqnvu4K1o+6SagEPda4hzQfKNKhY1mh5RBdk6W3Me27Xuoa4B/jtUN31exaXKWtYnM3dtP31qaiAtmfnbpQD4XrE9O/k6NJP8I1Oz+3t+8HK+upRT8LAcWPdN1CtMnTxwpk6+0B4/KfmCIXILUW7EJ1T7k/KHz+LnEsoTZYGA3UxCW/c6+FFNg9CX6g8SGUdtsC2k+MhNvWH3GoT4KrbXohANQE8EJxvEIkMw+jDnfiUG6gitaOUDWdhUSuLcYJixZEAXg8+BRzDbb83XbulHktSbCvQUI4dA19LBAc03Ol7pUpZrGqfP+hp+oFcXYQJPJl5QKuIt6Q/STf5WxQcXZMO2FDGI1DRTv3IyX+1m+x8hrA6i9u+4lA0b53jB/8hVp/M7kUpVzfNHDh6ithVljQGWEeyMcoc0Wm9LjNp69tUGK/OEqd3DY+mJP21665kFJzq1bWTveQD0QjwgLY0wV7nGBHl5RrRhzKK+Q0RxeJC+UD8QwJsAaBQQhWxaYrt4YMtCyNZwEPq6RWOFOD84JfIrS2ZcZAj0iyaUQqJ8YuznRXOBNmvQoOVRrXO79EitIcvV2HXvJ8Fskdrv/b+ANRf+qmoV33kSdgGx41wEf53B5/ZIqKs36DRdSVMnyyS+IbvvZ71OonkF0qbVGkS4I844kMheX24fjq9vmMORcanmWwdACTjg5owET3X14RwRTh4Q1KwSSu2xv/eZj009VX7+KUaNHRaUwNdvpmkeQ4+jJbPX9ChTVR4fNOAloIuQ2f9zTR8P37a4cZwtKsTGGEzCMRZ68lcxaF213M+4FnAZAR1kseHUEHCdZib+NdOuqppHBYTV6pSg4MjGQtjSlpY8ogeyqlh3itn3tLEVjvtp7H67uIHblEaIqDxjpOeh16q2Fn8TeK+d9Ta4E7XRQ3lM/NWlCWgfTEUYpNqYvuDRxHVYFtqlhLXIMxcwgw7U37 JT/M46aS 6VwpG2fp2afROMWEkt6c0XkJ60WR8hkBIy/X5IkOjIzRY6WQXTZ1PDoRkQhcX6q6NdTmBi/AItfoB+Oo54GhP+o4+2+RkmDftIyxoP7mHhfe1ONFzy0Cq6y35vRMcwtQtv0Gsg501ah+c7eVzrrBq4JaDyyi6kMuFawD28lW3HJlqvhnJLdKw4syuL6tT8o6XAhVPD4hRZII3JeReNane3BDEVDmbQvukDdV/T4cGIFWy/wwDsjnIT25ox2JbqyyJzwKInirJ67PtRw0o+cW9Pv0CxFvu85NzLP3/Z+ZSlS83d1gRhD0CDIFpmpgrxTy3BBb+D+zXJLfstqwEixWN8myHJrRu65hdED9/ Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/24/26 23:04, Barry Song wrote: > On Wed, Jun 24, 2026 at 11:02 PM David Hildenbrand (Arm) > wrote: >> >> On 6/24/26 01:16, Barry Song (Xiaomi) wrote: >>> We always unconditionally drain the LRU before retrying anon folio >>> reuse in wp_can_reuse_anon_folio(). Instead, assume !LRU anon folios >>> are in lru_cache, and use the refcount to avoid many unnecessary LRU >>> drains. >>> >>> Acked-by: Shakeel Butt >>> Reviewed-by: Baoquan He >>> Signed-off-by: Barry Song (Xiaomi) >>> --- >>> mm/memory.c | 8 +++++++- >>> 1 file changed, 7 insertions(+), 1 deletion(-) >>> >>> diff --git a/mm/memory.c b/mm/memory.c >>> index ff338c2abe92..f6848f4234a6 100644 >>> --- a/mm/memory.c >>> +++ b/mm/memory.c >>> @@ -4193,12 +4193,18 @@ static bool wp_can_reuse_anon_folio(struct folio *folio, >>> */ >>> if (folio_test_ksm(folio) || folio_ref_count(folio) > 3) >>> return false; >>> - if (!folio_test_lru(folio)) >>> + if (!folio_test_lru(folio)) { >>> + /* >>> + * Assume folio is on lru_cache and holds a cache reference. >>> + */ >>> + if (folio_ref_count(folio) > 2 + folio_test_swapcache(folio)) >>> + return false; >> >> I'm not keen on making this function even uglier, so no, not like that. >> >> We have the earlier "folio_ref_count(folio) > 3" check. >> >> In which scenarios can you trigger this such that we would care? >> >> If the answer is "I don't know" there is no reason for a change. > > As I replied to Shakeel in the v1 discussion [1], this can avoid a > large number of drains, both during Ubuntu boot and under normal > workloads: > > "I booted the system into Ubuntu, and after is this with this patch only? > > boot completed I observed: > > wp_reuse_skipped_drain: 5542 > do_swap_skipped_drain: 0 > > Then I built the kernel in a 1GB memcg using zRAM swap, and observed: > > wp_reuse_skipped_drain: 25017 > do_swap_skipped_drain: 43595 This is all data that belongs into this patch description, not hidden somewhere on the internet :) And why do we care about local draining? This is not a drain-all. Really, this patch needs more motivation clearly spelled out. > > So in summary, even without swap-in we can save a significant number of > drains in wp_can_reuse_anon_folio(). With heavy swap-in workloads, most > of the savings come from avoiding drains in do_swap_page(), while we > still see a substantial number of skipped drains in wp_can_reuse_anon_folio()." > > This is exactly the case where folio_ref_count(folio) == 3 and > !folio_test_swapcache(folio). That is why checking > folio_ref_count(folio) > 3 does not help. diff --git a/mm/memory.c b/mm/memory.c index fec2e9f2858a6..153a6166beeb4 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -4181,6 +4181,9 @@ static bool __wp_can_reuse_large_anon_folio(struct folio *folio, static bool wp_can_reuse_anon_folio(struct folio *folio, struct vm_area_struct *vma) { + const bool in_lru_cache = !folio_test_lru(folio); + const bool in_swapcache = folio_test_swapcache(folio); + if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && folio_test_large(folio)) return __wp_can_reuse_large_anon_folio(folio, vma); @@ -4191,15 +4194,16 @@ static bool wp_can_reuse_anon_folio(struct folio *folio, * * KSM doesn't necessarily raise the folio refcount. */ - if (folio_test_ksm(folio) || folio_ref_count(folio) > 3) + if (folio_test_ksm(folio) || + folio_ref_count(folio) > 1 + in_lru_cache + in_swapcache) return false; - if (!folio_test_lru(folio)) + if (in_lru_cache) /* * We cannot easily detect+handle references from * remote LRU caches or references to LRU folios. */ lru_add_drain(); - if (folio_ref_count(folio) > 1 + folio_test_swapcache(folio)) + if (folio_ref_count(folio) > 1 + in_swapcache) return false; if (!folio_trylock(folio)) return false; ? But I also want to hear why we care about the local draining. -- Cheers, David