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 031924DA1F for ; Tue, 5 Mar 2024 09:11:14 +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=1709629876; cv=none; b=sdY0l84D9667+2vvQTcpRnjXlMmEMz2zM8Z4/P/Dvd+eryl8T4n8wJt8xUUmJFx57+X1dAbczrt3oC9HdxpbrR9VA4nXSTA4pfqNefxk4vThZ5ydxTDPKzfy4dL3IzA9CSTDTlIkDBmVJggbtK63pwRduA3OuFnzpn3Xs2RrPBw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709629876; c=relaxed/simple; bh=LWb4xFlvZM/PMrK9B+lfdajQrEyZarnZ/3xa3Zq4WPA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=qtwNXUMhoyV1avLXJ6cfU0GrtY4LaSFkVskCTsfOnmONoH3H+gJBPqHuk4/uT+nXHmI97zuBQD8uVUhSfRzRxKT6ILtDBBPxsCfj6uBTbXUtoifvq7O715BjEAsutMQnX5twmwpNgdbUOaAPvvVMKd8Eoc3EcLs6HyXG1WkV1pE= 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 D70E51FB; Tue, 5 Mar 2024 01:11:50 -0800 (PST) Received: from [10.57.68.162] (unknown [10.57.68.162]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id E45F63F762; Tue, 5 Mar 2024 01:11:11 -0800 (PST) Message-ID: <0a644230-f7a8-4091-9d00-ded6c8c3fc19@arm.com> Date: Tue, 5 Mar 2024 09:11:10 +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: [RFC PATCH] mm: hold PTL from the first PTE while reclaiming a large folio Content-Language: en-GB To: Barry Song <21cnbao@gmail.com> Cc: akpm@linux-foundation.org, linux-mm@kvack.org, david@redhat.com, chrisl@kernel.org, yuzhao@google.com, hanchuanhua@oppo.com, linux-kernel@vger.kernel.org, willy@infradead.org, ying.huang@intel.com, xiang@kernel.org, mhocko@suse.com, shy828301@gmail.com, wangkefeng.wang@huawei.com, Barry Song , Hugh Dickins References: <20240304103757.235352-1-21cnbao@gmail.com> <706b7129-85f6-4470-9fd9-f955a8e6bd7c@arm.com> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 05/03/2024 09:08, Barry Song wrote: > On Tue, Mar 5, 2024 at 9:54 PM Ryan Roberts wrote: >> >> On 04/03/2024 21:57, Barry Song wrote: >>> On Tue, Mar 5, 2024 at 1:21 AM Ryan Roberts wrote: >>>> >>>> Hi Barry, >>>> >>>> On 04/03/2024 10:37, Barry Song wrote: >>>>> From: Barry Song >>>>> >>>>> page_vma_mapped_walk() within try_to_unmap_one() races with other >>>>> PTEs modification such as break-before-make, while iterating PTEs >>>>> of a large folio, it will only begin to acquire PTL after it gets >>>>> a valid(present) PTE. break-before-make intermediately sets PTEs >>>>> to pte_none. Thus, a large folio's PTEs might be partially skipped >>>>> in try_to_unmap_one(). >>>> >>>> I just want to check my understanding here - I think the problem occurs for >>>> PTE-mapped, PMD-sized folios as well as smaller-than-PMD-size large folios? Now >>>> that I've had a look at the code and have a better understanding, I think that >>>> must be the case? And therefore this problem exists independently of my work to >>>> support swap-out of mTHP? (From your previous report I was under the impression >>>> that it only affected mTHP). >>> >>> I think this affects all large folios with PTEs entries more than 1. but hugeTLB >>> is handled as a whole in try_to_unmap_one and its rmap is removed all >>> together, i feel hugeTLB doesn't have this problem. >>> >>>> >>>> Its just that the problem is becoming more pronounced because with mTHP, >>>> PTE-mapped large folios are much more common? >>> >>> right. as now large folios become a more common case, and it is my case >>> running in millions of phones. >>> >>> BTW, I feel we can somehow learn from hugeTLB, for example, we can reclaim >>> all PTEs all together rather than iterating PTEs one by one. This will improve >>> performance. for example, a batched >>> set_ptes_to_swap_entries() >>> { >>> } >>> then we only need to loop once for a large folio, right now we are looping >>> nr_pages times. >> >> You still need a pte-pte loop somewhere. In hugetlb's case it's in the arch >> implementation. HugeTLB ptes are all a fixed size for a given VMA, which makes >> things a bit easier too, whereas in the regular mm, they are now a variable size. >> >> David and I introduced folio_pte_batch() to help gather batches of ptes, and it >> uses the contpte bit to avoid iterating over intermediate ptes. And I'm adding >> swap_pte_batch() which does a similar thing for swap entry batching in v4 of my >> swap-out series. >> >> For your set_ptes_to_swap_entries() example, I'm not sure what it would do other >> than loop over the PTEs setting an incremented swap entry to each one? How is >> that more performant? > > right now, while (page_vma_mapped_walk(&pvmw)) will loop nr_pages for each > PTE, if each PTE, we do lots of checks within the loop. > > by implementing set_ptes_to_swap_entries(), we can iterate once for > page_vma_mapped_walk(), after folio_pte_batch() has confirmed > the large folio is completely mapped, we set nr_pages swap entries > all together. > > we are replacing > > for(i=0;i { > lots of checks; > clear PTEn > set PTEn to swap > } OK so you are effectively hoisting "lots of checks" out of the loop? > > by > > if (large folio && folio_pte_batch() == nr_pages) > set_ptes_to_swap_entries(). > >> > > Thanks, > Ryan