Re: [PATCH v10 00/12] LUF(Lazy Unmap Flush) reducing tlb numbers over 90%

Byungchul Park
2024-05-27T04:02:57Z

On Mon, May 27, 2024 at 11:10:15AM +0800, Huang, Ying wrote:
> Byungchul Park <byungchul@sk.com> writes:
>
> > On Fri, May 24, 2024 at 10:16:39AM -0700, Dave Hansen wrote:
> >> On 5/9/24 23:51, Byungchul Park wrote:
> >> > To achieve that:
> >> >
> >> > 1. For the folios that map only to non-writable tlb entries, prevent
> >> > tlb flush during unmapping but perform it just before the folios
> >> > actually become used, out of buddy or pcp.
> >>
> >> Is this just _pure_ unmapping (like MADV_DONTNEED), or does it apply to
> >> changing the memory map, like munmap() itself?
> >
> > I think it can be applied to any unmapping of ro ones but LUF for now is
> > working only with unmapping during folio migrion and reclaim.
> >
> >> > 2. When any non-writable ptes change to writable e.g. through fault
> >> > handler, give up luf mechanism and perform tlb flush required
> >> > right away.
> >> >
> >> > 3. When a writable mapping is created e.g. through mmap(), give up
> >> > luf mechanism and perform tlb flush required right away.
> >>
> >> Let's say you do this:
> >>
> >> fd = open("/some/file", O_RDONLY);
> >> ptr1 = mmap(-1, size, PROT_READ, ..., fd, ...);
> >> foo1 = *ptr1;
> >>
> >> You now have a read-only PTE pointing to the first page of /some/file.
> >> Let's say try_to_unmap() comes along and decides it can_luf_folio().
> >> The page gets pulled out of the page cache and freed, the PTE is zeroed.
> >> But the TLB is never flushed.
> >>
> >> Now, someone does:
> >>
> >> fd2 = open("/some/other/file", O_RDONLY);
> >> ptr2 = mmap(ptr1, size, PROT_READ, MAP_FIXED, fd, ...);
> >> foo2 = *ptr2;
> >>
> >> and they overwrite the old VMA. Does foo2 have the contents of the new
> >> "/some/other/file" or the old "/some/file"? How does the new mmap()
> >
> > Good point. It should've give up LUF at the 2nd mmap() in this case.
> > I will fix it by introducing a new flag in task_struct indicating if LUF
> > has left stale maps for the task so that LUF can give up and flush right
> > away in mmap().
> >
> >> know that there was something to flush?
> >>
> >> BTW, the same thing could happen without a new mmap(). Someone could
> >> modify the file in the middle, maybe even from another process.
> >
> > Thank you for the pointing out. I will fix it too by introducing a new
> > flag in inode or something to make LUF aware if updating the file has
> > been tried so that LUF can give up and flush right away in the case.
> >
> > Plus, I will add another give-up at code changing the permission of vma
> > to writable.
>
> I guess that you need a framework similar as
> "flush_tlb_batched_pending()" to deal with interaction with other TLB
> related operations.

Thank you. I will check it.

Byungchul

> --
> Best Regards,
> Huang, Ying
>
> > Thank you very much.
> >
> > Byungchul
> >
> >> fd = open("/some/file", O_RDONLY);
> >> ptr1 = mmap(-1, size, PROT_READ, ..., fd, ...);
> >> foo1 = *ptr1;
> >> // LUF happens here
> >> // "/some/file" changes
> >> foo2 = *ptr1; // Does this see the change?