[PATCH v5 7/9] mm/mremap: Move TLB flush outside page table lock

[Liam Howlett]

* Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com> [210521 08:51]:
> On 5/21/21 11:43 AM, Linus Torvalds wrote:
> > On Thu, May 20, 2021 at 5:03 PM Aneesh Kumar K.V
> > <aneesh.kumar at linux.ibm.com> wrote:
> > > 
> > > On 5/21/21 8:10 AM, Linus Torvalds wrote:
> > > > 
> > > > So mremap does need to flush the TLB before releasing the page table
> > > > lock, because that's the lifetime boundary for the page that got
> > > > moved.
> > > 
> > > How will we avoid that happening with
> > > c49dd340180260c6239e453263a9a244da9a7c85 /
> > > 2c91bd4a4e2e530582d6fd643ea7b86b27907151 . The commit improves mremap
> > > performance by moving level3/level2 page table entries. When doing so we
> > > are not holding level 4 ptl lock (pte_lock()). But rather we are holding
> > > pmd_lock or pud_lock(). So if we move pages around without holding the
> > > pte lock, won't the above issue happen even if we do a tlb flush with
> > > holding pmd lock/pud lock?
> > 
> > Hmm. Interesting.
> > 
> > Your patch (to flush the TLB after clearing the old location, and
> > before inserting it into the new one) looks like an "obvious" fix.
> > 
> > But I'm putting that "obvious" in quotes, because I'm now wondering if
> > it actually fixes anything.
> > 
> > Lookie here:
> > 
> >   - CPU1 does a mremap of a pmd or pud.
> > 
> >      It clears the old pmd/pud, flushes the old TLB range, and then
> > inserts the pmd/pud at the new location.
> > 
> >   - CPU2 does a page shrinker, which calls try_to_unmap, which calls
> > try_to_unmap_one.
> > 
> > These are entirely asynchronous, because they have no shared lock. The
> > mremap uses the pmd lock, the try_to_unmap_one() does the rmap walk,
> > which does the pte lock.
> > 
> > Now, imagine that the following ordering happens with the two
> > operations above, and a CPU3 that does accesses:
> > 
> >   - CPU2 follows (and sees) the old page tables in the old location and
> > the took the pte lock
> > 
> >   - the mremap on CPU1 starts - cleared the old pmd, flushed the tlb,
> > *and* inserts in the new place.
> > 
> >   - a user thread on CPU3 accesses the new location and fills the TLB
> > of the *new* address

mremap holds the mmap_sem in write mode as well, doesn't it?  How is the user thread
getting the new location?

> > 
> >   - only now does CPU2 get to the "pte_get_and_clear()" to remove one page
> > 
> >   - CPU2 does a TLB flush and frees the page
> > 
> > End result:
> > 
> >   - both CPU1 _and_ CPU2 have flushed the TLB.
> > 
> >   - but both flushed the *OLD* address
> > 
> >   - the page is freed
> > 
> >   - CPU3 still has the stale TLB entry pointing to the page that is now
> > free and might be reused for something else
> > 
> > Am I missing something?
> > 
> 
> That is a problem. With that it looks like CONFIG_HAVE_MOVE_PMD/PUD is
> broken? I don't see an easy way to fix this?
> 
> -aneesh
> 
>