Date:   Thu, 20 May 2021 12:21:02 -0700
From:   Mina Almasry <almasrymina@...gle.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Peter Xu <peterx@...hat.com>, Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm, hugetlb: fix resv_huge_pages underflow on UFFDIO_COPY

On Thu, May 20, 2021 at 12:18 PM Mina Almasry <almasrymina@...gle.com> wrote:
>
> On Thu, May 13, 2021 at 5:14 PM Mike Kravetz <mike.kravetz@...cle.com> wrote:
> >
> > On 5/13/21 4:49 PM, Mina Almasry wrote:
> > > On Thu, May 13, 2021 at 4:43 PM Mina Almasry <almasrymina@...gle.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.
>

I should also mention, without a prior remove_huge_page() this code
path works fine, so it seems the fact that the reservation is consumed
before causes trouble, but I'm not sure why yet.

> > --
> > Mike Kravetz

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