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

[Zhaoyang Huang]

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

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;
     }
}