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Date: Tue, 6 Dec 2022 11:58:09 -0800
From: Mike Kravetz <mike.kravetz@...cle.com>
To: Peter Xu <peterx@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
James Houghton <jthoughton@...gle.com>,
Jann Horn <jannh@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Andrea Arcangeli <aarcange@...hat.com>,
Rik van Riel <riel@...riel.com>,
Nadav Amit <nadav.amit@...il.com>,
Miaohe Lin <linmiaohe@...wei.com>,
Muchun Song <songmuchun@...edance.com>,
David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 09/10] mm/hugetlb: Make page_vma_mapped_walk() safe to
pmd unshare
On 12/06/22 12:43, Peter Xu wrote:
> On Tue, Dec 06, 2022 at 12:39:53PM -0500, Peter Xu wrote:
> > On Tue, Dec 06, 2022 at 09:10:00AM -0800, Mike Kravetz wrote:
> > > On 12/05/22 15:52, Mike Kravetz wrote:
> > > > On 11/29/22 14:35, Peter Xu wrote:
> > > > > Since page_vma_mapped_walk() walks the pgtable, it needs the vma lock
> > > > > to make sure the pgtable page will not be freed concurrently.
> > > > >
> > > > > Signed-off-by: Peter Xu <peterx@...hat.com>
> > > > > ---
> > > > > include/linux/rmap.h | 4 ++++
> > > > > mm/page_vma_mapped.c | 5 ++++-
> > > > > 2 files changed, 8 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> > > > > index bd3504d11b15..a50d18bb86aa 100644
> > > > > --- a/include/linux/rmap.h
> > > > > +++ b/include/linux/rmap.h
> > > > > @@ -13,6 +13,7 @@
> > > > > #include <linux/highmem.h>
> > > > > #include <linux/pagemap.h>
> > > > > #include <linux/memremap.h>
> > > > > +#include <linux/hugetlb.h>
> > > > >
> > > > > /*
> > > > > * The anon_vma heads a list of private "related" vmas, to scan if
> > > > > @@ -408,6 +409,9 @@ static inline void page_vma_mapped_walk_done(struct page_vma_mapped_walk *pvmw)
> > > > > pte_unmap(pvmw->pte);
> > > > > if (pvmw->ptl)
> > > > > spin_unlock(pvmw->ptl);
> > > > > + /* This needs to be after unlock of the spinlock */
> > > > > + if (is_vm_hugetlb_page(pvmw->vma))
> > > > > + hugetlb_vma_unlock_read(pvmw->vma);
> > > > > }
> > > > >
> > > > > bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw);
> > > > > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> > > > > index 93e13fc17d3c..f94ec78b54ff 100644
> > > > > --- a/mm/page_vma_mapped.c
> > > > > +++ b/mm/page_vma_mapped.c
> > > > > @@ -169,10 +169,13 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> > > > > if (pvmw->pte)
> > > > > return not_found(pvmw);
> > > > >
> > > > > + hugetlb_vma_lock_read(vma);
> > > > > /* when pud is not present, pte will be NULL */
> > > > > pvmw->pte = huge_pte_offset(mm, pvmw->address, size);
> > > > > - if (!pvmw->pte)
> > > > > + if (!pvmw->pte) {
> > > > > + hugetlb_vma_unlock_read(vma);
> > > > > return false;
> > > > > + }
> > > > >
> > > > > pvmw->ptl = huge_pte_lock(hstate, mm, pvmw->pte);
> > > > > if (!check_pte(pvmw))
> > > >
> > > > I think this is going to cause try_to_unmap() to always fail for hugetlb
> > > > shared pages. See try_to_unmap_one:
> > > >
> > > > while (page_vma_mapped_walk(&pvmw)) {
> > > > ...
> > > > if (folio_test_hugetlb(folio)) {
> > > > ...
> > > > /*
> > > > * To call huge_pmd_unshare, i_mmap_rwsem must be
> > > > * held in write mode. Caller needs to explicitly
> > > > * do this outside rmap routines.
> > > > *
> > > > * We also must hold hugetlb vma_lock in write mode.
> > > > * Lock order dictates acquiring vma_lock BEFORE
> > > > * i_mmap_rwsem. We can only try lock here and fail
> > > > * if unsuccessful.
> > > > */
> > > > if (!anon) {
> > > > VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
> > > > if (!hugetlb_vma_trylock_write(vma)) {
> > > > page_vma_mapped_walk_done(&pvmw);
> > > > ret = false;
> > > > }
> > > >
> > > >
> > > > Can not think of a great solution right now.
> > >
> > > Thought of this last night ...
> > >
> > > Perhaps we do not need vma_lock in this code path (not sure about all
> > > page_vma_mapped_walk calls). Why? We already hold i_mmap_rwsem.
> >
> > Exactly. The only concern is when it's not in a rmap.
> >
> > I'm actually preparing something that adds a new flag to PVMW, like:
> >
> > #define PVMW_HUGETLB_NEEDS_LOCK (1 << 2)
> >
> > But maybe we don't need that at all, since I had a closer look the only
> > outliers of not using a rmap is:
> >
> > __replace_page
> > write_protect_page
> >
> > I'm pretty sure ksm doesn't have hugetlb involved, then the other one is
> > uprobe (uprobe_write_opcode). I think it's the same. If it's true, we can
> > simply drop this patch. Then we also have hugetlb_walk and the lock checks
> > there guarantee that we're safe anyways.
> >
> > Potentially we can document this fact, which I also attached a comment
> > patch just for it to be appended to the end of the patchset.
> >
> > Mike, let me know what do you think.
> >
> > Andrew, if this patch to be dropped then the last patch may not cleanly
> > apply. Let me know if you want a full repost of the things.
>
> The document patch that can be appended to the end of this series attached.
> I referenced hugetlb_walk() so it needs to be the last patch.
>
> --
> Peter Xu
Agree with dropping this patch and adding the document patch below.
Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
Also, happy we have the warnings in place to catch incorrect locking.
--
Mike Kravetz
> From 754c2180804e9e86accf131573cbd956b8c62829 Mon Sep 17 00:00:00 2001
> From: Peter Xu <peterx@...hat.com>
> Date: Tue, 6 Dec 2022 12:36:04 -0500
> Subject: [PATCH] mm/hugetlb: Document why page_vma_mapped_walk() is safe to
> walk
> Content-type: text/plain
>
> Taking vma lock here is not needed for now because all potential hugetlb
> walkers here should have i_mmap_rwsem held. Document the fact.
>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
> mm/page_vma_mapped.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index e97b2e23bd28..2e59a0419d22 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -168,8 +168,14 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> /* The only possible mapping was handled on last iteration */
> if (pvmw->pte)
> return not_found(pvmw);
> -
> - /* when pud is not present, pte will be NULL */
> + /*
> + * NOTE: we don't need explicit lock here to walk the
> + * hugetlb pgtable because either (1) potential callers of
> + * hugetlb pvmw currently holds i_mmap_rwsem, or (2) the
> + * caller will not walk a hugetlb vma (e.g. ksm or uprobe).
> + * When one day this rule breaks, one will get a warning
> + * in hugetlb_walk(), and then we'll figure out what to do.
> + */
> pvmw->pte = hugetlb_walk(vma, pvmw->address, size);
> if (!pvmw->pte)
> return false;
> --
> 2.37.3
>
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