Re: [PATCH] mm, hugetlb: fix resv_huge_pages underflow on UFFDIO_COPY

[Mina Almasry <almasrymina@google.com>]

On Thu, May 13, 2021 at 5:14 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 5/13/21 4:49 PM, Mina Almasry wrote:
> > On Thu, May 13, 2021 at 4:43 PM Mina Almasry <almasrymina@google.com> wrote:
> >>
> >> When hugetlb_mcopy_atomic_pte() is called with:
> >> - mode==MCOPY_ATOMIC_NORMAL and,
> >> - we already have a page in the page cache corresponding to the
> >> associated address,
> >>
> >> We will allocate a huge page from the reserves, and then fail to insert it
> >> into the cache and return -EEXIST. In this case, we need to return -EEXIST
> >> without allocating a new page as the page already exists in the cache.
> >> Allocating the extra page causes the resv_huge_pages to underflow temporarily
> >> until the extra page is freed.
> >>
> >> To fix this we check if a page exists in the cache, and allocate it and
> >> insert it in the cache immediately while holding the lock. After that we
> >> copy the contents into the page.
> >>
> >> As a side effect of this, pages may exist in the cache for which the
> >> copy failed and for these pages PageUptodate(page) == false. Modify code
> >> that query the cache to handle this correctly.
> >>
> >
> > To be honest, I'm not sure I've done this bit correctly. Please take a
> > look and let me know what you think. It may be too overly complicated
> > to have !PageUptodate() pages in the cache and ask the rest of the
> > code to handle that edge case correctly, but I'm not sure how else to
> > fix this issue.
> >
>
> I think you just moved the underflow from hugetlb_mcopy_atomic_pte to
> hugetlb_no_page.  Why?
>
> Consider the case where there is only one reserve left and someone does
> the MCOPY_ATOMIC_NORMAL for the address.  We will allocate the page and
> consume the reserve (reserve count == 0) and insert the page into the
> cache.  Now, if the copy_huge_page_from_user fails we must drop the
> locks/fault mutex to do the copy.  While locks are dropped, someone
> faults on the address and ends up in hugetlb_no_page.  The page is in
> the cache but not up to date, so we go down the allocate new page path
> and will decrement the reserve count again to cause underflow.
>
> How about this approach?
> - Keep the check for hugetlbfs_pagecache_present in hugetlb_mcopy_atomic_pte
>   that you added.  That will catch the race where the page was added to
>   the cache before entering the routine.
> - With the above check in place, we only need to worry about the case
>   where copy_huge_page_from_user fails and we must drop locks.  In this
>   case we:
>   - Free the page previously allocated.
>   - Allocate a 'temporary' huge page without consuming reserves.  I'm
>     thinking of something similar to page migration.
>   - Drop the locks and let the copy_huge_page_from_user be done to the
>     temporary page.
>   - When reentering hugetlb_mcopy_atomic_pte after dropping locks (the
>     *pagep case) we need to once again check
>     hugetlbfs_pagecache_present.
>   - We then try to allocate the huge page which will consume the
>     reserve.  If successful, copy contents of temporary page to newly
>     allocated page.  Free temporary page.
>
> There may be issues with this, and I have not given it deep thought.  It
> does abuse the temporary huge page concept, but perhaps no more than
> page migration.  Things do slow down if the extra page allocation and
> copy is required, but that would only be the case if copy_huge_page_from_user
> needs to be done without locks.  Not sure, but hoping that is rare.

Just following up this a bit: I've implemented this approach locally,
and with it it's passing the test as-is. When I hack the code such
that the copy in hugetlb_mcopy_atomic_pte() always fails, I run into
this edge case, which causes resv_huge_pages to underflow again (this
time permemantly):

- hugetlb_no_page() is called on an index and a page is allocated and
inserted into the cache consuming the reservation.
- remove_huge_page() is called on this index and the page is removed from cache.
- hugetlb_mcopy_atomic_pte() is called on this index, we do not find
the page in the cache and we trigger this code patch and the copy
fails.
- The allocations in this code path seem to double consume the
reservation and resv_huge_pages underflows.

I'm looking at this edge case to understand why a prior
remove_huge_page() causes my code to underflow resv_huge_pages.

> --
> Mike Kravetz