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

[David Hildenbrand]

On 03.06.24 15:23, 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.
>
> > 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.

Fully agreed!

> > 	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.
> 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
> mode that enforced a rule that said:
>
>    1. A LUF'd PTE can't be rewritten until after a luf_flush() occurs

I was playing with the idea of using a PTE marker. Then it's clear for 
munmap/mremap/page faults that there is an outstanding flush required. 
the alternative might be a VMA flag, but that's harder to actually 
enforce an invariant.

>    2. A LUF'd page's position in the page cache can't be replaced until
>       after a luf_flush()

That's the most tricky bit. I think these are the VFS concerns like

1) Page migration/reclaim ends up freeing the old page. TLB not flushed.
2) write() to the new page / write from other process to the new page
3) CPU reads stale content from old page

PTE markers can't handle that.

--
Cheers,

David / dhildenb