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

[Byungchul Park <byungchul@sk.com>]

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?