Re: [PATCH] fs: buffer: check huge page size instead of single page for invalidatepage

[Yang Shi]

On Mon, Sep 20, 2021 at 3:35 PM Yang Shi <shy828301@xxxxxxxxx> wrote:
>
> On Mon, Sep 20, 2021 at 2:50 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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@xxxxxxxxxx/)
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.