From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7D5FF3FC5D8 for ; Fri, 26 Jun 2026 16:36:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782491763; cv=none; b=faxY6dCDybBMkr3zbtmF9ebkPsgAUzIQ+NarREVVUOyCpu3/oeHGHbscCD6HRxKZzgkG01OKaPL5GPKdY/X2D7T4OE87kO+S/upexAgkobrBv6sd0VaXUKSAgLjG38KUPG/kgxIahNL8GHXmo+RI83oy4zthlNN050NQlkDkTqc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782491763; c=relaxed/simple; bh=yTvo0jPglZd3hVYbjPYUPU4CmKnwOzuEcF2pqVbP77M=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Wf8uhS8xxJlrC5Namvv2gBc2Mrgj8cLbsE8zPlu1hAqlPUCsFfNCCwHk0JKvucpd0el8XcqovHEAtzWycxvHGW9u6178p76Fk/BAwBWUQaTWCMBDsKokzDQRvaA+q+kh6w3ZCuJVr+tnpC4sOzyo1cxKq2JypeFZCq1DQUtgrFg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=mQ3b33Rt; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="mQ3b33Rt" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4CFC61F000E9; Fri, 26 Jun 2026 16:35:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782491762; bh=qPvLxcG0grrH18rtYxN/ib3m110h2uI/oSNgSCAF3J0=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=mQ3b33RtK+P4LPpthlxcWBG+UXAE3+Pvfx34gbD3hy0zZCvd3BErhlyEEukdC9ZmN iiHyZT8uzmSzPZ5T79YLxLxHO6Wc1Gm65y/BcgLJ3IwN1duRiztdKAHBWeYeTK8XE0 Et6/NBK3v1kPTs6ujv1DHf/46UYsUfr1wN2KRH3QMEH1KvtIBg0DaGQ/c4ESBRAFnY RUDc4RLKRA/n8eY50ZjBgg6bBbPr1Bu4pS0RdjRRUCeRkMnWOyxRL2ugrI2zWuSC4M r58dHmNl2BW3NzVSvzhl6QIxOLrM2QNnr9qxOocSrnR7eFsZgr3A9XaOpkRGqRMj9X tm/ZZBtU0LGJQ== Message-ID: Date: Fri, 26 Jun 2026 18:35:55 +0200 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 4/4] mm: try to free swapcache for non-LRU folios To: Kairui Song , 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> <20260623231635.43086-5-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: 7bit >>> That changed that behavior such that we *must* now always fallback to do_wp_page(). >>> >>> What a mess (I didn't ack) > > That's awkward, my bad, terribly sorry about this :( > > I sincerely apologize for the oversight and the trouble this has caused. > All good, I was just surprised to see a previous optimization partially reverted without a clear reasoning :) Because it should have removed the handling in should_try_to_free_swap() as well. It's good that we are discussing it now! > I haven't seen any performance regression in any workload recently > though, or any correctness issue, perhaps the round trip of > do_wp_page wasn't that bad. It should still catch the reuse folios, > just more costly than doing things in-place. Right, do_wp_page() handles it, after the page was mapped. It adds some overhead, but fortunately no TLB flush if we're just upgrading write permissions. The optimization dates back to pre PageAnonExclusive handling. > > I think we should restore the original check first. We might also want to > avoid dropping the swap cache if the folio will not be reused, which > was discussed here: > > https://lore.kernel.org/linux-mm/CAMgjq7BDfvNXdWH0cqarsujjUn3i3tDDhDkmSg01TR4h-tDorQ@mail.gmail.com/ > > Maybe extracting some common part into a helper can help make this > cleaner. > >> >> So it seems we need a patch before patch 4 to fix the missed >> swapcache freeing on write faults, because the write bit has already >> been cleared by the time the folio is mapped and reused? >> >> diff --git a/mm/memory.c b/mm/memory.c >> index 14577c67c61a..e8fc5a86f6ea 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -4752,6 +4752,7 @@ static void check_swap_exclusive(struct folio >> *folio, swp_entry_t entry, >> */ >> vm_fault_t do_swap_page(struct vm_fault *vmf) >> { >> + enum fault_flag fault_flags = vmf->flags; >> struct vm_area_struct *vma = vmf->vma; >> struct folio *swapcache = NULL, *folio; >> struct page *page; >> @@ -5091,7 +5092,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >> * an additional reference to the folio. >> */ >> if (should_try_to_free_swap(si, folio, vma, nr_pages + >> - !folio_test_lru(folio), vmf->flags)) >> + !folio_test_lru(folio), fault_flags)) >> folio_free_swap(folio); >> >> folio_unlock(folio); >> >> >> Best Regards >> Barry > > Hi Barry, > > The problem is more than that, the `exclusive || folio_ref_count(folio) == 1` > in do_swap_page is also ineffective now. Exactly. If the roundtrip through do_wp_page() is good enough today, we can just do @@ -4512,7 +4516,6 @@ static vm_fault_t remove_device_exclusive_entry(struct vm_fault *vmf) static inline bool should_try_to_free_swap(struct swap_info_struct *si, struct folio *folio, struct vm_area_struct *vma, - unsigned int extra_refs, unsigned int fault_flags) { if (!folio_test_swapcache(folio)) @@ -4528,14 +4531,7 @@ static inline bool should_try_to_free_swap(struct swap_info_struct *si, if (mem_cgroup_swap_full(folio) || (vma->vm_flags & VM_LOCKED) || folio_test_mlocked(folio)) return true; - /* - * If we want to map a page that's in the swapcache writable, we - * have to detect via the refcount if we're really the exclusive - * user. Try freeing the swapcache to get rid of the swapcache - * reference only in case it's likely that we'll be the exclusive user. - */ - return (fault_flags & FAULT_FLAG_WRITE) && !folio_test_ksm(folio) && - folio_ref_count(folio) == (extra_refs + folio_nr_pages(folio)); + return false; } static vm_fault_t pte_marker_clear(struct vm_fault *vmf) @@ -5095,7 +5091,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) * Do it after mapping, so raced page faults will likely see the folio * in swap cache and wait on the folio lock. */ - if (should_try_to_free_swap(si, folio, vma, nr_pages, vmf->flags)) + if (should_try_to_free_swap(si, folio, vma, vmf->flags)) folio_free_swap(folio); folio_unlock(folio); But then, one question is whether we'd actually want to try removing the swapcache when we mapped the page writable (iow: exclusive)? @@ -4512,7 +4516,7 @@ static vm_fault_t remove_device_exclusive_entry(struct vm_fault *vmf) static inline bool should_try_to_free_swap(struct swap_info_struct *si, struct folio *folio, struct vm_area_struct *vma, - unsigned int extra_refs, + bool exclusive, unsigned int fault_flags) { if (!folio_test_swapcache(folio)) @@ -4529,13 +4533,11 @@ static inline bool should_try_to_free_swap(struct swap_info_struct *si, folio_test_mlocked(folio)) return true; /* - * If we want to map a page that's in the swapcache writable, we - * have to detect via the refcount if we're really the exclusive - * user. Try freeing the swapcache to get rid of the swapcache - * reference only in case it's likely that we'll be the exclusive user. + * We have an exclusive page that was mapped writable or will soon + * be mapped writable (as we are in a write fault). Let's just try + * to reclaim swap immediately. */ - return (fault_flags & FAULT_FLAG_WRITE) && !folio_test_ksm(folio) && - folio_ref_count(folio) == (extra_refs + folio_nr_pages(folio)); + return (fault_flags & FAULT_FLAG_WRITE) && exclusive; } static vm_fault_t pte_marker_clear(struct vm_fault *vmf) @@ -5095,7 +5097,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) * Do it after mapping, so raced page faults will likely see the folio * in swap cache and wait on the folio lock. */ - if (should_try_to_free_swap(si, folio, vma, nr_pages, vmf->flags)) + if (should_try_to_free_swap(si, folio, vma, exclusive, vmf->flags)) folio_free_swap(folio); folio_unlock(folio); -- Cheers, David