Re: [PATCH v2] mm/khugepaged: skip shmem with userfaultfd

[Matthew Wilcox <willy@infradead.org>]

On Tue, Feb 07, 2023 at 10:37:06AM +0900, David Stevens wrote:
> On Tue, Feb 7, 2023 at 6:50 AM Matthew Wilcox <willy@infradead.org> wrote:
> > On Mon, Feb 06, 2023 at 03:52:19PM -0500, Peter Xu wrote:
> > > The problem is khugepaged will release pgtable lock during collapsing, so
> > > AFAICT there can be a race where some other thread tries to insert pages
> > > into page cache in parallel with khugepaged right after khugepaged released
> > > the page cache lock.
> > >
> > > For example, it seems to me new page cache can be inserted when khugepaged
> > > is copying small page content to the new hpage.
> 
> This particular race can't happen with either patch, since the missing
> page cache entries are filled when we create the multi-index entry for
> hpage.

Can too.

        for (index = start; index < end; index++) {
...
                        if (xa_is_value(page) || !PageUptodate(page)) {
                                xas_unlock_irq(&xas);
                                /* swap in or instantiate fallocated page */
                                if (shmem_get_folio(mapping->host, index,
                                                &folio, SGP_NOALLOC)) {
                                        result = SCAN_FAIL;
                                        goto xa_unlocked;
                                }
...

So we start the iteration, and then a page fault happens in one of
the indices we've already examined, but we don't have the page on
the list.  It's a nice wide race too because we're bringing the
page in from swap.