From: Yang Shi <shy828301@gmail.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Hugh Dickins <hughd@google.com>,
cfijalkovich@google.com, Song Liu <song@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Hao Sun <sunhao.th@gmail.com>, Linux MM <linux-mm@kvack.org>,
Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Song Liu <songliubraving@fb.com>
Subject: Re: [PATCH] fs: buffer: check huge page size instead of single page for invalidatepage
Date: Mon, 11 Oct 2021 12:57:51 -0700 [thread overview]
Message-ID: <CAHbLzkrdXQfcudeeDHx8uUD55Rr=Aogi0pnQbBbP8bEZca8-7w@mail.gmail.com> (raw)
In-Reply-To: <CAHbLzkoXrVJOfOrNhd8nQFRPHhRVYfVYSgLAO3DO7ZmvaZtDVw@mail.gmail.com>
On Mon, Sep 20, 2021 at 3:35 PM Yang Shi <shy828301@gmail.com> wrote:
>
> On Mon, Sep 20, 2021 at 2:50 PM Matthew Wilcox <willy@infradead.org> 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 <willy@infradead.org> 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.
>
I think Hugh's skipping special file patch
(https://lore.kernel.org/linux-mm/a07564a3-b2fc-9ffe-3ace-3f276075ea5c@google.com/)
could fix this specific BUG report and seems like a more proper fix to
this.
However, it still doesn't make too much sense to have thp_size passed
to do_invalidatepage(), then have PAGE_SIZE hardcoded in a BUG
assertion IMHO. So it seems this patch is still useful because
block_invalidatepage() is called by a few filesystems as well, for
example, ext4. Or I'm wondering whether we should call
do_invalidatepage() for each subpage of THP in truncate_cleanup_page()
since private is for each subpage IIUC.
> 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.
next prev parent reply other threads:[~2021-10-11 19:58 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-17 20:57 [PATCH] fs: buffer: check huge page size instead of single page for invalidatepage Yang Shi
2021-09-18 0:07 ` Yang Shi
2021-09-19 14:40 ` Matthew Wilcox
2021-09-20 21:23 ` Yang Shi
2021-09-20 21:50 ` Matthew Wilcox
2021-09-20 22:35 ` Yang Shi
2021-10-11 19:57 ` Yang Shi [this message]
2021-10-20 23:38 ` Yang Shi
2021-10-20 23:49 ` Matthew Wilcox
2021-10-21 0:24 ` Yang Shi
2021-10-21 1:36 ` Matthew Wilcox
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