From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 81334C77B78 for ; Tue, 2 May 2023 23:41:00 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id E0D946B0071; Tue, 2 May 2023 19:40:59 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id DBE526B0072; Tue, 2 May 2023 19:40:59 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id CADCD6B0074; Tue, 2 May 2023 19:40:59 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) by kanga.kvack.org (Postfix) with ESMTP id 90D1C6B0071 for ; Tue, 2 May 2023 19:40:59 -0400 (EDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Transfer-Encoding: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description; bh=ixUAA5uNhzAoOQPplItQehmJAKqUj/F6wVFG7dDKyGE=; b=NN/kZVDqglULqT5zSsS1VhodAV aKhTZx9jkdKv7e9TJYl21DqMmeyd0Nr8RDdfm4IESQLh4/rFC4WQCMMHxR51/BJepYp9mbu93G65K gLlUU3nlqrvK8r6N9D4SJ0SGp/2wCzC29UAFmaOrli7gsefN3Wlw3XiBaF2/0r7AOHaF0YGMGjKje lbV6KW9MkdCwLXKxd1zkPT9DxPqMjjHHCcxH4vGjUG9hhW61rSGngnjW5NMlAxzLs82bMpzbwjy23 2D7p8emkLqCgtj7bsJbJj7RovfeP8mr0qB6SbhcmOwSOpLSV1z2AGupvV0gjPRVIT7K1e00Dx4xeY KJ9r20gA==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1ptzbd-008rny-Jj; Tue, 02 May 2023 23:40:25 +0000 Date: Wed, 3 May 2023 00:40:25 +0100 From: Matthew Wilcox To: Suren Baghdasaryan Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@suse.com, josef@toxicpanda.com, jack@suse.cz, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, michel@lespinasse.org, liam.howlett@oracle.com, jglisse@google.com, vbabka@suse.cz, minchan@google.com, dave@stgolabs.net, punit.agrawal@bytedance.com, lstoakes@gmail.com, hdanton@sina.com, apopple@nvidia.com, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH 1/3] mm: handle swap page faults under VMA lock if page is uncontended Message-ID: References: <20230501175025.36233-1-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: 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: On Tue, May 02, 2023 at 04:04:59PM -0700, Suren Baghdasaryan wrote: > On Tue, May 2, 2023 at 3:31 PM Matthew Wilcox wrote: > > > > On Tue, May 02, 2023 at 09:36:03AM -0700, Suren Baghdasaryan wrote: > > > On Tue, May 2, 2023 at 8:03 AM Matthew Wilcox wrote: > > > > > > > > On Mon, May 01, 2023 at 10:04:56PM -0700, Suren Baghdasaryan wrote: > > > > > On Mon, May 1, 2023 at 8:22 PM Matthew Wilcox wrote: > > > > > > > > > > > > On Mon, May 01, 2023 at 07:30:13PM -0700, Suren Baghdasaryan wrote: > > > > > > > On Mon, May 1, 2023 at 7:02 PM Matthew Wilcox wrote: > > > > > > > > > > > > > > > > On Mon, May 01, 2023 at 10:50:23AM -0700, Suren Baghdasaryan wrote: > > > > > > > > > +++ b/mm/memory.c > > > > > > > > > @@ -3711,11 +3711,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > > > > > > > > > if (!pte_unmap_same(vmf)) > > > > > > > > > goto out; > > > > > > > > > > > > > > > > > > - if (vmf->flags & FAULT_FLAG_VMA_LOCK) { > > > > > > > > > - ret = VM_FAULT_RETRY; > > > > > > > > > - goto out; > > > > > > > > > - } > > > > > > > > > - > > > > > > > > > entry = pte_to_swp_entry(vmf->orig_pte); > > > > > > > > > if (unlikely(non_swap_entry(entry))) { > > > > > > > > > if (is_migration_entry(entry)) { > > > > > > > > > > > > > > > > You're missing the necessary fallback in the (!folio) case. > > > > > > > > swap_readpage() is synchronous and will sleep. > > > > > > > > > > > > > > True, but is it unsafe to do that under VMA lock and has to be done > > > > > > > under mmap_lock? > > > > > > > > > > > > ... you were the one arguing that we didn't want to wait for I/O with > > > > > > the VMA lock held? > > > > > > > > > > Well, that discussion was about waiting in folio_lock_or_retry() with > > > > > the lock being held. I argued against it because currently we drop > > > > > mmap_lock lock before waiting, so if we don't drop VMA lock we would > > > > > be changing the current behavior which might introduce new > > > > > regressions. In the case of swap_readpage and swapin_readahead we > > > > > already wait with mmap_lock held, so waiting with VMA lock held does > > > > > not introduce new problems (unless there is a need to hold mmap_lock). > > > > > > > > > > That said, you are absolutely correct that this situation can be > > > > > improved by dropping the lock in these cases too. I just didn't want > > > > > to attack everything at once. I believe after we agree on the approach > > > > > implemented in https://lore.kernel.org/all/20230501175025.36233-3-surenb@google.com > > > > > for dropping the VMA lock before waiting, these cases can be added > > > > > easier. Does that make sense? > > > > > > > > OK, I looked at this path some more, and I think we're fine. This > > > > patch is only called for SWP_SYNCHRONOUS_IO which is only set for > > > > QUEUE_FLAG_SYNCHRONOUS devices, which are brd, zram and nvdimms > > > > (both btt and pmem). So the answer is that we don't sleep in this > > > > path, and there's no need to drop the lock. > > > > > > Yes but swapin_readahead does sleep, so I'll have to handle that case > > > too after this. > > > > Sleeping is OK, we do that in pXd_alloc()! Do we block on I/O anywhere > > in swapin_readahead()? It all looks like async I/O to me. > > Hmm. I thought that we have synchronous I/O in the following paths: > swapin_readahead()->swap_cluster_readahead()->swap_readpage() > swapin_readahead()->swap_vma_readahead()->swap_readpage() > but just noticed that in both cases swap_readpage() is called with the > synchronous parameter being false. So you are probably right here... > Does that mean swapin_readahead() might return a page which does not > have its content swapped-in yet? That's my understanding. In that case it's !uptodate and still locked. The folio_lock_or_retry() will wait for the read to complete unless we've told it we'd rather retry.