On 3/21/18 10:29 AM, Matthew Wilcox wrote:
On Wed, Mar 21, 2018 at 09:31:22AM -0700, Yang Shi wrote:
On 3/21/18 6:08 AM, Michal Hocko wrote:
Yes, this definitely sucks. One way to work that around is to split the
unmap to two phases. One to drop all the pages. That would only need
mmap_sem for read and then tear down the mapping with the mmap_sem for
write. This wouldn't help for parallel mmap_sem writers but those really
need a different approach (e.g. the range locking).
page fault might sneak in to map a page which has been unmapped before?
range locking should help a lot on manipulating small sections of a large
mapping in parallel or multiple small mappings. It may not achieve too much
for single large mapping.
I don't think we need range locking. What if we do munmap this way:
Take the mmap_sem for write
Find the VMA
If the VMA is large(*)
Mark the VMA as deleted
Drop the mmap_sem
zap all of the entries
Take the mmap_sem
Else
zap all of the entries
Continue finding VMAs
Drop the mmap_sem
Now we need to change everywhere which looks up a VMA to see if it needs
to care the the VMA is deleted (page faults, eg will need to SIGBUS; mmap
does not care; munmap will need to wait for the existing munmap operation
The other question is why munmap need wait? If the other parallel munmap
finds the vma has been marked as "deleted", it just need return 0 as it
doesn't find vma.
Currently do_munmap() does the below logic:
vma = find_vma(mm, start);
if (!vma)
return 0;
Yang
to complete), but it gives us the atomicity, at least on a per-VMA basis.
We could also do:
Take the mmap_sem for write
Mark all VMAs in the range as deleted & modify any partial VMAs
Drop mmap_sem
zap pages from deleted VMAs
That would give us the same atomicity that we have today.
Deleted VMAs would need a pointer to a completion, so operations that
need to wait can queue themselves up. I'd recommend we use the low bit
of vm_file and treat it as a pointer to a struct completion if set.
