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SEZPR02MB5614.apcprd02.prod.outlook.com (2603:1096:101:39::5) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.7362.35; Wed, 13 Mar 2024 02:21:47 +0000 Received: from TYZPR02MB7988.apcprd02.prod.outlook.com ([fe80::9aeb:5c84:e751:1bad]) by TYZPR02MB7988.apcprd02.prod.outlook.com ([fe80::9aeb:5c84:e751:1bad%5]) with mapi id 15.20.7362.031; Wed, 13 Mar 2024 02:21:47 +0000 Message-ID: <24dc6251-8582-790f-bbd3-465deed946f5@oppo.com> Date: Wed, 13 Mar 2024 10:21:42 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.12.0 Subject: Re: [RFC PATCH v3 3/5] mm: swap: make should_try_to_free_swap() support large-folio To: Ryan Roberts , Barry Song <21cnbao@gmail.com>, akpm@linux-foundation.org, linux-mm@kvack.org Cc: chengming.zhou@linux.dev, chrisl@kernel.org, david@redhat.com, hannes@cmpxchg.org, kasong@tencent.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, 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 v4/MoGMr 7jB3SHIMCRrzhYgUIee3XxN+YCohTbQCoWCeb91XmgmwvHbtSaZGAVtwFGyZhAIgcvWXqT/HJz3fVksIpEoTTNpFujQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: hi, Ryan Roberts 在 2024/3/12 20:34, Ryan Roberts 写道: > On 04/03/2024 08:13, Barry Song wrote: >> From: Chuanhua Han >> >> should_try_to_free_swap() works with an assumption that swap-in is always done >> at normal page granularity, aka, folio_nr_pages = 1. To support large folio >> swap-in, this patch removes the assumption. >> >> Signed-off-by: Chuanhua Han >> Co-developed-by: Barry Song >> Signed-off-by: Barry Song >> Acked-by: Chris Li >> --- >> mm/memory.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/mm/memory.c b/mm/memory.c >> index abd4f33d62c9..e0d34d705e07 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -3837,7 +3837,7 @@ static inline bool should_try_to_free_swap(struct folio *folio, >> * reference only in case it's likely that we'll be the exlusive user. >> */ >> return (fault_flags & FAULT_FLAG_WRITE) && !folio_test_ksm(folio) && >> - folio_ref_count(folio) == 2; >> + folio_ref_count(folio) == (1 + folio_nr_pages(folio)); > I don't think this is correct; one reference has just been added to the folio in > do_swap_page(), either by getting from swapcache (swap_cache_get_folio()) or by > allocating. If it came from the swapcache, it could be a large folio, because we > swapped out a large folio and never removed it from swapcache. But in that case, > others may have partially mapped it, so the refcount could legitimately equal > the number of pages while still not being exclusively mapped. > > I'm guessing this logic is trying to estimate when we are likely exclusive so > that we remove from swapcache (release ref) and can then reuse rather than CoW > the folio? The main CoW path currently CoWs page-by-page even for large folios, > and with Barry's recent patch, even the last page gets copied. So not sure what > this change is really trying to achieve? > First, if it is a large folio in the swap cache, then its refcont is at least folio_nr_pages(folio) :   For example, in add_to_swap_cache path: int add_to_swap_cache(struct folio *folio, swp_entry_t entry,                         gfp_t gfp, void **shadowp) {         struct address_space *address_space = swap_address_space(entry);         pgoff_t idx = swp_offset(entry);         XA_STATE_ORDER(xas, &address_space->i_pages, idx, folio_order(folio));         unsigned long i, nr = folio_nr_pages(folio); <---         void *old;         ...         folio_ref_add(folio, nr); <---         folio_set_swapcache(folio);         ... } * Then in the do_swap_page path: * if (should_try_to_free_swap(folio, vma, vmf->flags))         folio_free_swap(folio); * * It also indicates that only folio in the swap cache will call folio_free_swap * to delete it from the swap cache, So I feel like this patch is necessary!? 😁 >> } >> >> static vm_fault_t pte_marker_clear(struct vm_fault *vmf) Thanks, Chuanhua