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.
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.
