Re: summarize all information again at bottom//reply: reply: [PATCH] mm: fix a race scenario in folio_isolate_lru

[Matthew Wilcox]

On Thu, Mar 28, 2024 at 12:03:02PM +0800, Zhaoyang Huang wrote:
> On Thu, Mar 28, 2024 at 11:18 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, Mar 28, 2024 at 09:27:31AM +0800, Zhaoyang Huang wrote:
> > > ok, I missed the refcnt from alloc_pages. However, I still think it is
> > > a bug to call readahead_folio in read_pages as the refcnt obtained by
> > > alloc_pages should be its final guard which is paired to the one which
> > > checked in shrink_folio_list->__remove_mapping->folio_ref_freeze(2)(this
> > > 2 represent alloc_pages & page cache). If we removed this one without
> >
> > __remove_mapping()  requires that the caller holds the folio locked.
> > Since the readahead code unlocks the folio, __remove_mapping() cannot
> > be run because the caller of __remove_mapping() will wait for the folio
> > lock.
> repost the whole timing sequence to make it more clear and fix
> incorrect description of previous feedback

I can't understand what you think the problem is here.  Please try
again.

> Follow the refcount through.
> 
> In page_cache_ra_unbounded():
> 
>                 folio = filemap_alloc_folio(gfp_mask, 0);
> (folio has refcount 1)
>                 ret = filemap_add_folio(mapping, folio, index + i, gfp_mask);
> (folio has refcount 2, PG_lru)
> 
> Then we call read_pages()
> First we call ->readahead() which for some reason stops early.
> Then we call readahead_folio() which calls folio_put()
> (folio has refcount 1)
> Then we call folio_get()
> (folio has refcount 2)
> Then we call filemap_remove_folio()
> (folio has refcount 1)
> Then we call folio_unlock()
> Then we call folio_put()
> 
> Amending steps for previous timing sequence below where [1] races with
> [2] that has nothing to do with __remove_mapping(). IMO, no file_folio
> should be freed by folio_put as the refcnt obtained by alloc_pages
> keep it always imbalanced until shrink_folio_list->__remove_mapping,
> where the folio_ref_freeze(2) implies the refcnt of alloc_pages and
> isolation should be the last two. release_pages is a special scenario
> that the refcnt of alloc_pages is freed implicitly in
> delete_from_page_cache_batch->filemap_free_folio.
> 
>     folio_put()
>     {
>          if(folio_put_test_zero())
> *** we should NOT be here as the refcnt of alloc_pages should NOT be dropped ***
>          if (folio_test_lru())
> ***  preempted here with refcnt == 0 and pass PG_lru check ***
> [1]
>          lruvec_del_folio()
> Then thread_isolate call folio_isolate_lru()
>       folio_isolate_lru()
>       {
>          folio_test_clear_lru()
>          folio_get()
> [2]
>          lruvec_del_folio()
>       }
> --------------------------------------------------------------------------------------------
> shrink_folio_list()
> {
>     __remove_mapping()
>     {
>         refcount = 1 + folio_nr_pages;
> *** the refcount = 1 + 1 implies there should be only the refcnt of
> alloc_pages and previous isolation for a no-busy folio as all PTE has
> gone***
>         if (!folio_ref_freeze(refcount))
>              goto keeplock;
>      }
> }