From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id E55962568 for ; Fri, 1 Mar 2024 17:06:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709312774; cv=none; b=FjswUlggc/DvfXv3CjUE98vOae/cRv5S7HKtsnVtjYVAMUeUq9LDdSZ49tchhkFFd81EDuhRfP9/54eBB6AubfViIwETcJbmBJQpQdD2I4WaNL/L7kBPSmYbEA6frxaDLgMWADt4wC3t4aiV4Aoh9UvqGIkX9HCDP2rCnIbGG+E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709312774; c=relaxed/simple; bh=As0naI/KPotwoAB4PDT0RMD0J2b2/+8uxVGUb5E7Lk0=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=fiPaA8ryk7+RE+13wcwmGqLXVHE/0m0ZwBs/9JG5XSxXQzrtSJpILbrV6UsTuZUfoGNfhHHKAwdnvEQVAAak7suuyph5OM3lAeBitW1sxjik4P9Jw8kFNYYyrASS8IzABgO5olOj5fsQ1Z/95uGEDDwDzb8D4LhjeEBR6sbYpYQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 5AFFF1FB; Fri, 1 Mar 2024 09:06:48 -0800 (PST) Received: from [10.57.68.58] (unknown [10.57.68.58]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 083873F6C4; Fri, 1 Mar 2024 09:06:07 -0800 (PST) Message-ID: Date: Fri, 1 Mar 2024 17:06:05 +0000 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 v3 1/4] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags Content-Language: en-GB From: Ryan Roberts To: Matthew Wilcox Cc: David Hildenbrand , Andrew Morton , Huang Ying , Gao Xiang , Yu Zhao , Yang Shi , Michal Hocko , Kefeng Wang , linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <6541e29b-f25a-48b8-a553-fd8febe85e5a@redhat.com> <2934125a-f2e2-417c-a9f9-3cb1e074a44f@redhat.com> <049818ca-e656-44e4-b336-934992c16028@arm.com> <4a73b16e-9317-477a-ac23-8033004b0637@arm.com> <1195531c-d985-47e2-b7a2-8895fbb49129@redhat.com> <5ebac77a-5c61-481f-8ac1-03bc4f4e2b1d@arm.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 01/03/2024 16:44, Ryan Roberts wrote: > On 01/03/2024 16:31, Matthew Wilcox wrote: >> On Fri, Mar 01, 2024 at 04:27:32PM +0000, Ryan Roberts wrote: >>> I've implemented the batching as David suggested, and I'm pretty confident it's >>> correct. The only problem is that during testing I can't provoke the code to >>> take the path. I've been pouring through the code but struggling to figure out >>> under what situation you would expect the swap entry passed to >>> free_swap_and_cache() to still have a cached folio? Does anyone have any idea? >>> >>> This is the original (unbatched) function, after my change, which caused David's >>> concern that we would end up calling __try_to_reclaim_swap() far too much: >>> >>> int free_swap_and_cache(swp_entry_t entry) >>> { >>> struct swap_info_struct *p; >>> unsigned char count; >>> >>> if (non_swap_entry(entry)) >>> return 1; >>> >>> p = _swap_info_get(entry); >>> if (p) { >>> count = __swap_entry_free(p, entry); >>> if (count == SWAP_HAS_CACHE) >>> __try_to_reclaim_swap(p, swp_offset(entry), >>> TTRS_UNMAPPED | TTRS_FULL); >>> } >>> return p != NULL; >>> } >>> >>> The trouble is, whenever its called, count is always 0, so >>> __try_to_reclaim_swap() never gets called. >>> >>> My test case is allocating 1G anon memory, then doing madvise(MADV_PAGEOUT) over >>> it. Then doing either a munmap() or madvise(MADV_FREE), both of which cause this >>> function to be called for every PTE, but count is always 0 after >>> __swap_entry_free() so __try_to_reclaim_swap() is never called. I've tried for >>> order-0 as well as PTE- and PMD-mapped 2M THP. >> >> I think you have to page it back in again, then it will have an entry in >> the swap cache. Maybe. I know little about anon memory ;-) > > Ahh, I was under the impression that the original folio is put into the swap > cache at swap out, then (I guess) its removed once the IO is complete? I'm sure > I'm miles out... what exactly is the lifecycle of a folio going through swap out? > > I guess I can try forking after swap out, then fault it back in in the child and > exit. Then do the munmap in the parent. I guess that could force it? Thanks for > the tip - I'll have a play. That has sort of solved it, the only problem now is that all the folios in the swap cache are small (because I don't have Barry's large swap-in series). So really I need to figure out how to avoid removing the folio from the cache in the first place... > >> >> If that doesn't work, perhaps use tmpfs, and use some memory pressure to >> force that to swap? >> >>> I'm guessing the swapcache was already reclaimed as part of MADV_PAGEOUT? I'm >>> using a block ram device as my backing store - I think this does synchronous IO >>> so perhaps if I have a real block device with async IO I might have more luck? >>> Just a guess... >>> >>> Or perhaps this code path is a corner case? In which case, perhaps its not worth >>> adding the batching optimization after all? >>> >>> Thanks, >>> Ryan >>> >