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

Dave Hansen
2024-05-24T17:16:44Z

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?

> 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()
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.

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?