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.
