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

[Kiryl Shutsemau]

On Tue, Oct 21, 2025 at 07:16:33AM +0100, Kiryl Shutsemau wrote:
> 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.

Any comments?

Jan, I remember you worked a lot on making GUP semantics sanish for file
pages. Any clues if I imagine a problem here?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov