Re: [PATCH mmotm] mm/thp: refix __split_huge_pmd_locked() for migration PMD

[Hugh Dickins]

On Thu, 3 Mar 2022, Yang Shi wrote:
> On Wed, Mar 2, 2022 at 5:43 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> >
> > Migration entries do not contribute to a page's reference count: move
> > __split_huge_pmd_locked()'s page_ref_add() into pmd_migration's else
> > block (along with the page_count() check - a page is quite likely to
> > to have reference count frozen to 0 when a migration entry is found).
> >
> > This will fix a very rare anonymous memory leak, after a split_huge_pmd()
> > raced with an anon split_huge_page() or an anon THP migrate_pages(): since
> > the wrongly raised refcount stopped the page (perhaps small, perhaps huge,
> > depending on when the race hit) from ever being freed.  At first I thought
> > there were worse risks, from prematurely unfreezing a frozen page: but now
> > think that would only affect page cache pages, which do not come this way
> > (except for anonymous pages in swap cache, perhaps).
> 
> Thanks for catching this. I agree there may be anon memory leak due to
> bumped refcount. But I don't think it could affect page cache page
> since that code (bumping refcount) is never called for page cache page
> IIUC.

Yes, that's what I meant by unfreezing "would only affect page cache pages,
which do not come this way".  But then remembered that anonymous pages in
swap cache also have non-NULL page_mapping(), and involve freezing in the
same way that page cache pages do.  Now, offhand I forget the status of
hugepage swap, so maybe that's impossible, hence "perhaps".  Easier to
fix than to think through all the ramifications of not fixing, as usual.

> 
> The patch looks good to me. Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>

Thanks,
Hugh