Re: [RFC, PATCH 0/2] Large folios vs. SIGBUS semantics

[Kiryl Shutsemau]

On Tue, Oct 21, 2025 at 10:28:02AM +1100, Dave Chinner wrote:
> In critical paths like truncate, correctness and safety come first.
> Performance is only a secondary consideration.  The overlap of
> mmap() and truncate() is an area where we have had many, many bugs
> and, at minimum, the current POSIX behaviour largely shields us from
> serious stale data exposure events when those bugs (inevitably)
> occur.

How do you prevent writes via GUP racing with truncate()?

Something like this:

	CPU0				CPU1
fd = open("file")
p = mmap(fd)
whatever_syscall(p)
  get_user_pages(p, &page)
  				truncate("file");
  <write to page>
  put_page(page);

The GUP can pin a page in the middle of a large folio well beyond the
truncation point. The folio will not be split on truncation due to the
elevated pin.

I don't think this issue can be fundamentally fixed as long as we allow
GUP for file-backed memory.

If the filesystem side cannot handle a non-zeroed tail of a large folio,
this SIGBUS semantics only hides the issue instead of addressing it.

And the race above does not seem to be far-fetched to me.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov