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Mon, 20 Sep 2021 15:35:20 -0700 (PDT) MIME-Version: 1.0 References: <20210917205731.262693-1-shy828301@gmail.com> In-Reply-To: From: Yang Shi Date: Mon, 20 Sep 2021 15:35:08 -0700 Message-ID: Subject: Re: [PATCH] fs: buffer: check huge page size instead of single page for invalidatepage To: Matthew Wilcox Cc: Hugh Dickins , cfijalkovich@google.com, song@kernel.org, Andrew Morton , Hao Sun , Linux MM , Linux FS-devel Mailing List , Linux Kernel Mailing List , Song Liu Content-Type: text/plain; charset="UTF-8" X-Stat-Signature: miieiuje5swqr9mjzqoz5744uinpzfbh Authentication-Results: imf29.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=prKje2XM; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf29.hostedemail.com: domain of shy828301@gmail.com designates 209.85.208.54 as permitted sender) smtp.mailfrom=shy828301@gmail.com X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: AC3869000254 X-HE-Tag: 1632177321-435517 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 Mon, Sep 20, 2021 at 2:50 PM Matthew Wilcox wrote: > > On Mon, Sep 20, 2021 at 02:23:41PM -0700, Yang Shi wrote: > > On Sun, Sep 19, 2021 at 7:41 AM Matthew Wilcox wrote: > > > > > > On Fri, Sep 17, 2021 at 05:07:03PM -0700, Yang Shi wrote: > > > > > The debugging showed the page passed to invalidatepage is a huge page > > > > > and the length is the size of huge page instead of single page due to > > > > > read only FS THP support. But block_invalidatepage() would throw BUG if > > > > > the size is greater than single page. > > > > > > Things have already gone wrong before we get to this point. See > > > do_dentry_open(). You aren't supposed to be able to get a writable file > > > descriptor on a file which has had huge pages added to the page cache > > > without the filesystem's knowledge. That's the problem that needs to > > > be fixed. > > > > I don't quite understand your point here. Do you mean do_dentry_open() > > should fail for such cases instead of truncating the page cache? > > No, do_dentry_open() should have truncated the page cache when it was > called and found that there were THPs in the cache. Then khugepaged > should see that someone has the file open for write and decline to create > new THPs. So it shouldn't be possible to get here with THPs in the cache. AFAICT, it does so. In do_dentry_open(): /* * XXX: Huge page cache doesn't support writing yet. Drop all page * cache for this file before processing writes. */ if (f->f_mode & FMODE_WRITE) { /* * Paired with smp_mb() in collapse_file() to ensure nr_thps * is up to date and the update to i_writecount by * get_write_access() is visible. Ensures subsequent insertion * of THPs into the page cache will fail. */ smp_mb(); if (filemap_nr_thps(inode->i_mapping)) truncate_pagecache(inode, 0); } In khugepaged: filemap_nr_thps_inc(mapping); /* * Paired with smp_mb() in do_dentry_open() to ensure * i_writecount is up to date and the update to nr_thps is * visible. Ensures the page cache will be truncated if the * file is opened writable. */ smp_mb(); if (inode_is_open_for_write(mapping->host)) { result = SCAN_FAIL; __mod_lruvec_page_state(new_page, NR_FILE_THPS, -nr); filemap_nr_thps_dec(mapping); goto xa_locked; } But I'm not quite sure if there is any race condition.