Re: [PATCH v11 09/12] mm: implement LUF(Lazy Unmap Flush) defering tlb flush when folios get unmapped

[David Hildenbrand]

On 04.06.24 06:43, Byungchul Park wrote:
> On Tue, Jun 04, 2024 at 10:53:48AM +0900, Byungchul Park wrote:
> > On Mon, Jun 03, 2024 at 06:23:46AM -0700, Dave Hansen wrote:
> > > On 6/3/24 02:35, Byungchul Park wrote:
> > > ...> In luf's point of view, the points where the deferred flush should be
> > > > performed are simply:
> > > >
> > > > 	1. when changing the vma maps, that might be luf'ed.
> > > > 	2. when updating data of the pages, that might be luf'ed.
> > >
> > > It's simple, but the devil is in the details as always.
> >
> > Agree with that.
> >
> > > > All we need to do is to indentify the points:
> > > >
> > > > 	1. when changing the vma maps, that might be luf'ed.
> > > >
> > > > 	   a) mmap and munmap e.i. fault handler or unmap_region().
> > > > 	   b) permission to writable e.i. mprotect or fault handler.
> > > > 	   c) what I'm missing.
> > >
> > > I'd say it even more generally: anything that installs a PTE which is
> > > inconsistent with the original PTE.  That, of course, includes writes.
> > > But it also includes crazy things that we do like uprobes.  Take a look
> > > at __replace_page().
> > >
> > > I think the page_vma_mapped_walk() checks plus the ptl keep LUF at bay
> > > there.  But it needs some really thorough review.
> > >
> > > But the bigger concern is that, if there was a problem, I can't think of
> > > a systematic way to find it.
> > >
> > > > 	2. when updating data of the pages, that might be luf'ed.
> > > >
> > > > 	   a) updating files through vfs e.g. file_end_write().
> > > > 	   b) updating files through writable maps e.i. 1-a) or 1-b).
> > > > 	   c) what I'm missing.
> > >
> > > Filesystems or block devices that change content without a "write" from
> > > the local system.  Network filesystems and block devices come to mind.
> >
> > AFAIK, every network filesystem eventully "updates" its connected local
> > filesystem.  It could be still handled at the point where updating the
> > local file system.
> >
> > > I honestly don't know what all the rules are around these, but they
> > > could certainly be troublesome.
> > >
> > > There appear to be some interactions for NFS between file locking and
> > > page cache flushing.
> > >
> > > But, stepping back ...
> > >
> > > I'd honestly be a lot more comfortable if there was even a debugging LUF
> >
> > I'd better provide a method for better debugging.  Lemme know whatever
> > it is we need.
> >
> > > mode that enforced a rule that said:
>
> Do you means a debugging mode that can WARN or inform the situation that
> we don't want?  If yes, sure.  Now that I get this, I will re-read all
> you guys' talk.

In my opinion either for debugging, or for actually enforcing it at 
runtime. Whatever the cost of that would be needs to be determined.

--
Cheers,

David / dhildenb