Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

[Peter Xu]

On Thu, Aug 10, 2023 at 07:47:35PM +0200, David Hildenbrand wrote:
> On 10.08.23 19:15, Peter Xu wrote:
> > On Thu, Aug 10, 2023 at 11:48:27AM +0100, Ryan Roberts wrote:
> > > > For PTE-mapped THP, it might be a bit bigger noise, although I doubt it is
> > > > really significant (judging from my experience on managing PageAnonExclusive
> > > > using set_bit/test_bit/clear_bit when (un)mapping anon pages).
> > > > 
> > > > As folio_add_file_rmap_range() indicates, for PTE-mapped THPs we should be
> > > > batching where possible (and Ryan is working on some more rmap batching).
> > > 
> > > Yes, I've just posted [1] which batches the rmap removal. That would allow you
> > > to convert the per-page atomic_dec() into a (usually) single per-large-folio
> > > atomic_sub().
> > > 
> > > [1] https://lore.kernel.org/linux-mm/20230810103332.3062143-1-ryan.roberts@xxxxxxx/
> > 
> > Right, that'll definitely make more sense, thanks for the link; I'd be very
> > happy to read more later (finally I got some free time recently..).  But
> > then does it mean David's patch can be attached at the end instead of
> > proposed separately and early?
> 
> Not in my opinion. Batching rmap makes sense even without this change, and
> this change makes sense even without batching.
> 
> > 
> > I was asking mostly because I read it as a standalone patch first, and
> > honestly I don't know the effect.  It's based on not only the added atomic
> > ops itself, but also the field changes.
> > 
> > For example, this patch moves Hugh's _nr_pages_mapped into the 2nd tail
> > page, I think it means for any rmap change of any small page of a huge one
> > we'll need to start touching one more 64B cacheline on x86.  I really have
> > no idea what does it mean for especially a large SMP: see 292648ac5cf1 on
> > why I had an impression of that.  But I've no enough experience or clue to
> > prove it a problem either, maybe would be interesting to measure the time
> > needed for some pte-mapped loops?  E.g., something like faulting in a thp,
> 
> Okay, so your speculation right now is:
> 
> 1) The change in cacheline might be problematic.
> 
> 2) The additional atomic operation might be problematic.
> 
> > then measure the split (by e.g. mprotect() at offset 1M on a 4K?) time it
> > takes before/after this patch.
> 
> I can certainly try getting some numbers on that. If you're aware of other
> micro-benchmarks that would likely notice slower pte-mapping of THPs, please
> let me know.

Thanks.

> 
> > 
> > When looking at this, I actually found one thing that is slightly
> > confusing, not directly relevant to your patch, but regarding the reuse of
> > tail page 1 on offset 24 bytes.  Current it's Hugh's _nr_pages_mapped,
> > and you're proposing to replace it with the total mapcount:
> > 
> >          atomic_t   _nr_pages_mapped;     /*    88     4 */
> > 
> > Now my question is.. isn't byte 24 of tail page 1 used for keeping a
> > poisoned mapping?  See prep_compound_tail() where it has:
> > 
> > 	p->mapping = TAIL_MAPPING;
> > 
> > While here mapping is, afaict, also using offset 24 of the tail page 1:
> > 
> >          struct address_space * mapping;  /*    24     8 */
> > 
> > I hope I did a wrong math somewhere, though.
> > 
> 
> I think your math is correct.
> 
> prep_compound_head() is called after prep_compound_tail(), so
> prep_compound_head() wins.
> 
> In __split_huge_page_tail() there is a VM_BUG_ON_PAGE() that explains the
> situation:
> 
> /* ->mapping in first and second tail page is replaced by other uses */
> VM_BUG_ON_PAGE(tail > 2 && page_tail->mapping != TAIL_MAPPING,
> 	       page_tail);
> 
> Thanks for raising that, I had to look into that myself.

It's so confusing so I did try to document them a bit myself, then I found
maybe I should just post a patch for it and I just did:

https://lore.kernel.org/r/20230810204944.53471-1-peterx@xxxxxxxxxx

It'll conflict with yours, but I marked RFC so it's never anything urgent,
but maybe that'll be helpful already for you to introduce any new fields
like total_mapcounts.

AFAICS if that patch was all correct (while I'm not yet sure..), you can
actually fit your new total mapcount field into page 1 so even avoid the
extra cacheline access.  You can have a look: the trick is refcount for
tail page 1 is still seems to be free on 32 bits (if that was your worry
before).  Then it'll be very nice if to keep Hugh's counter all in tail 1.

-- 
Peter Xu