Re: [RFC PATCH] mm,mremap: Bail out earlier in mremap_to under map pressure

[Kirill A. Shutemov]

On Mon, Feb 25, 2019 at 12:46:46PM +0100, Vlastimil Babka wrote:
> On 2/22/19 2:01 PM, Kirill A. Shutemov wrote:
> > On Thu, Feb 21, 2019 at 09:54:06AM +0100, Oscar Salvador wrote:
> >> When using mremap() syscall in addition to MREMAP_FIXED flag,
> >> mremap() calls mremap_to() which does the following:
> >>
> >> 1) unmaps the destination region where we are going to move the map
> >> 2) If the new region is going to be smaller, we unmap the last part
> >>    of the old region
> >>
> >> Then, we will eventually call move_vma() to do the actual move.
> >>
> >> move_vma() checks whether we are at least 4 maps below max_map_count
> >> before going further, otherwise it bails out with -ENOMEM.
> >> The problem is that we might have already unmapped the vma's in steps
> >> 1) and 2), so it is not possible for userspace to figure out the state
> >> of the vma's after it gets -ENOMEM, and it gets tricky for userspace
> >> to clean up properly on error path.
> >>
> >> While it is true that we can return -ENOMEM for more reasons
> >> (e.g: see may_expand_vm() or move_page_tables()), I think that we can
> >> avoid this scenario in concret if we check early in mremap_to() if the
> >> operation has high chances to succeed map-wise.
> >>
> >> Should not be that the case, we can bail out before we even try to unmap
> >> anything, so we make sure the vma's are left untouched in case we are likely
> >> to be short of maps.
> >>
> >> The thumb-rule now is to rely on the worst-scenario case we can have.
> >> That is when both vma's (old region and new region) are going to be split
> >> in 3, so we get two more maps to the ones we already hold (one per each).
> >> If current map count + 2 maps still leads us to 4 maps below the threshold,
> >> we are going to pass the check in move_vma().
> >>
> >> Of course, this is not free, as it might generate false positives when it is
> >> true that we are tight map-wise, but the unmap operation can release several
> >> vma's leading us to a good state.
> >>
> >> Because of that I am sending this as a RFC.
> >> Another approach was also investigated [1], but it may be too much hassle
> >> for what it brings.
> > 
> > I believe we don't need the check in move_vma() with this patch. Or do we?
> 
> move_vma() can be also called directly from SYSCALL_DEFINE5(mremap) for
> the non-MMAP_FIXED case. So unless there's further refactoring, the
> check is still needed.

Okay, makes sense.

> >>
> >> [1] https://lore.kernel.org/lkml/20190219155320.tkfkwvqk53tfdojt@xxxxxxxxxxxx/
> >>
> >> Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
> 
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
 Kirill A. Shutemov