Re: [PATCH v4 08/14] mm/gup: grab head page refcount once for group of subpages

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Oct 13, 2021 at 08:18:08PM +0100, Joao Martins wrote:
> On 10/13/21 18:41, Jason Gunthorpe wrote:
> > On Mon, Oct 11, 2021 at 04:53:29PM +0100, Joao Martins wrote:
> >> On 10/8/21 12:54, Jason Gunthorpe wrote:
> > 
> >>> The only optimization that might work here is to grab the head, then
> >>> compute the extent of tail pages and amalgamate them. Holding a ref on
> >>> the head also secures the tails.
> >>
> >> How about pmd_page(orig) / pud_page(orig) like what the rest of hugetlb/thp
> >> checks do? i.e. we would pass pmd_page(orig)/pud_page(orig) to __gup_device_huge()
> >> as an added @head argument. While keeping the same structure of counting tail pages
> >> between @addr .. @end if we have a head page.
> > 
> > The right logic is what everything else does:
> > 
> > 	page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
> > 	refs = record_subpages(page, addr, end, pages + *nr);
> > 	head = try_grab_compound_head(pud_page(orig), refs, flags);
> > 
> > If you can use this, or not, depends entirely on answering the
> > question of why does  __gup_device_huge() exist at all.
> > 
> So for device-dax it seems to be an untackled oversight[*], probably
> inherited from when fsdax devmap was introduced. What I don't know
> is the other devmap users :(

devmap generic infrastructure waits until all page refcounts go to
zero, and it should wait until any fast GUP is serialized as part of
the TLB shootdown - otherwise it is leaking access to the memory it
controls beyond it's shutdown

So, I don't think the different devmap users can break this?

> > This I don't fully know:
> > 
> > 1) As discussed quite a few times now, the entire get_dev_pagemap
> >    stuff looks usless and should just be deleted. If you care about
> >    optimizing this I would persue doing that as it will give the
> >    biggest single win.
> 
> I am not questioning the well-deserved improvement -- but from a pure
> optimization perspective the get_dev_pagemap() cost is not
> visible and quite negligeble.

You are doing large enough GUPs then that the expensive xarray seach
is small compared to the rest?

> > 2) It breaks up the PUD/PMD into tail pages and scans them all
> >    Why? Can devmap combine multiple compound_head's into the same PTE?
> 
> I am not aware of any other usage of compound pages for devmap struct pages
> than this series. At least I haven't seen device-dax or fsdax using
> this.

Let me ask this question differently, is this assertion OK?

--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -808,8 +808,13 @@ static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
        }
 
        entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));
-       if (pfn_t_devmap(pfn))
+       if (pfn_t_devmap(pfn)) {
+               struct page *pfn_to_page(pfn);
+
+               WARN_ON(compound_head(page) != page);
+               WARN_ON(compound_order(page) != PMD_SHIFT);
                entry = pmd_mkdevmap(entry);
+       }
        if (write) {
                entry = pmd_mkyoung(pmd_mkdirty(entry));
                entry = maybe_pmd_mkwrite(entry, vma);

(and same for insert_pfn_pud)

You said it is OK for device/dax/device.c?

And not for fs/dax.c?


> Unless HMM does this stuff, or some sort of devmap page migration? P2PDMA
> doesn't seem to be (yet?) caught by any of the GUP path at least before
> Logan's series lands. Or am I misunderstanding things here?

Of the places that call the insert_pfn_pmd/pud call chains I only see
device/dax/device.c and fs/dax.c as being linked to devmap. So other
devmap users don't use this stuff.

> I was changing __gup_device_huge() with similar to the above, but yeah
> it follows that algorithm as inside your do { } while() (thanks!). I can
> turn __gup_device_huge() into another (renamed to like try_grab_pages())
> helper and replace the callsites of gup_huge_{pud,pmd} for the THP/hugetlbfs
> equivalent handling.

I suppose it should be some #define because the (pmd_val != orig) logic
is not sharable

But, yes, a general call that the walker should make at any level to
record a pfn -> npages range efficiently.

> I think the right answer is "depends on the devmap" type. device-dax with
> PMD/PUDs (i.e. 2m pagesize PMEM or 1G pagesize pmem) works with the same
> rules as hugetlbfs. fsdax not so much (as you say above) but it would
> follow up changes to perhaps adhere to similar scheme (not exactly sure
> how do deal with holes). HMM I am not sure what the rules are there.
> P2PDMA seems not applicable?

I would say, not "depends on the devmap", but what are the rules for
calling insert_pfn_pmd in the first place.

If users are allowed the create pmds that span many compound_head's
then we need logic as I showed. Otherwise we do not.

And I would document this relationship in the GUP side "This do/while
is required because insert_pfn_pmd/pud() is used with compound pages
smaller than the PUD/PMD size" so it isn't so confused with just
"devmap"

Jason