Re: reply: [PATCH] mm: fix a race scenario in folio_isolate_lru

[Matthew Wilcox]

On Mon, Mar 18, 2024 at 01:37:04AM +0000, 黄朝阳 (Zhaoyang Huang) wrote:
> >On Sun, Mar 17, 2024 at 12:07:40PM +0800, Zhaoyang Huang wrote:
> >> Could it be this scenario, where folio comes from pte(thread 0), local
> >> fbatch(thread 1) and page cache(thread 2) concurrently and proceed
> >> intermixed without lock's protection? Actually, IMO, thread 1 also
> >> could see the folio with refcnt==1 since it doesn't care if the page
> >> is on the page cache or not.
> >>
> >> madivise_cold_and_pageout does no explicit folio_get thing since the
> >> folio comes from pte which implies it has one refcnt from pagecache
> >
> >Mmm, no.  It's implicit, but madvise_cold_or_pageout_pte_range()
> >does guarantee that the folio has at least one refcount.
> >
> >Since we get the folio from vm_normal_folio(vma, addr, ptent); we know that
> >there is at least one mapcount on the folio.  refcount is always >= mapcount.
> >Since we hold pte_offset_map_lock(), we know that mapcount (and therefore
> >refcount) cannot be decremented until we call pte_unmap_unlock(), which we
> >don't do until we have called folio_isolate_lru().
> >
> >Good try though, took me a few minutes of looking at it to convince myself that
> >it was safe.
> >
> >Something to bear in mind is that if the race you outline is real, failing to hold a
> >refcount on the folio leaves the caller susceptible to the
> >VM_BUG_ON_FOLIO(!folio_ref_count(folio), folio); if the other thread calls
> >folio_put().
> Resend the chart via outlook.
> I think the problem rely on an special timing which is rare, I would like to list them below in timing sequence.
> 
> 1. thread 0 calls folio_isolate_lru with refcnt == 1

(i assume you mean refcnt == 2 here, otherwise none of this makes sense)

> 2. thread 1 calls release_pages with refcnt == 2.(IMO, it could be 1 as release_pages doesn't care if the folio is used by page cache or fs)
> 3. thread 2 decrease refcnt to 1 by calling filemap_free_folio.(as I mentioned in 2, thread 2 is not mandatary here)
> 4. thread 1 calls folio_put_testzero and pass.(lruvec->lock has not been take here)

But there's already a bug here.

Rearrange the order of this:

2. thread 1 calls release_pages with refcount == 2 (decreasing refcount to 1)
3. thread 2 decrease refcount to 0 by calling filemap_free_folio
1. thread 0 calls folio_isolate_lru() and hits the BUG().

> 5. thread 0 clear folio's PG_lru by calling folio_test_clear_lru. The folio_get behind has no meaning there.
> 6. thread 1 failed in folio_test_lru and leave the folio on the LRU.
> 7. thread 1 add folio to pages_to_free wrongly which could break the LRU's->list and will have next folio experience list_del_invalid
> 
> #thread 0(madivise_cold_and_pageout)        #1(lru_add_drain->fbatch_release_pages)       #2(read_pages->filemap_remove_folios)
> refcnt == 1(represent page cache)             refcnt==2(another one represent LRU)          folio comes from page cache

This is still illegible.  Try it this way:

Thread 0	Thread 1	Thread 2
madvise_cold_or_pageout_pte_range
		lru_add_drain
		fbatch_release_pages
				read_pages
				filemap_remove_folio

Some accuracy in your report would also be appreciated.  There's no
function called madivise_cold_and_pageout, nor is there a function called
filemap_remove_folios().  It's a little detail, but it's annoying for
me to try to find which function you're actually referring to.  I have
to guess, and it puts me in a bad mood.

At any rate, these three functions cannot do what you're proposing.
In read_page(), when we call filemap_remove_folio(), the folio in
question will not have the uptodate flag set, so can never have been
put in the page tables, so cannot be found by madvise().

Also, as I said in my earlier email, madvise_cold_or_pageout_pte_range()
does guarantee that the refcount on the folio is held and can never
decrease to zero while folio_isolate_lru() is running.  So that's two
ways this scenario cannot happen.