From: Peter Xu <peterx@redhat.com>
To: Alistair Popple <apopple@nvidia.com>
Cc: linux-mm@kvack.org, akpm@linux-foundation.org,
nouveau@lists.freedesktop.org, bskeggs@redhat.com,
rcampbell@nvidia.com, linux-doc@vger.kernel.org,
jhubbard@nvidia.com, bsingharora@gmail.com,
linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
hch@infradead.org, jglisse@redhat.com, willy@infradead.org,
jgg@nvidia.com, hughd@google.com, Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v9 07/10] mm: Device exclusive memory access
Date: Fri, 28 May 2021 09:11:25 -0400 [thread overview]
Message-ID: <YLDr/RyAdUR4q0kk@t490s> (raw)
In-Reply-To: <2243324.CkbYuGXDfH@nvdebian>
On Fri, May 28, 2021 at 11:48:40AM +1000, Alistair Popple wrote:
[...]
> > > > > + while (page_vma_mapped_walk(&pvmw)) {
> > > > > + /* Unexpected PMD-mapped THP? */
> > > > > + VM_BUG_ON_PAGE(!pvmw.pte, page);
> > > > > +
> > > > > + if (!pte_present(*pvmw.pte)) {
> > > > > + ret = false;
> > > > > + page_vma_mapped_walk_done(&pvmw);
> > > > > + break;
> > > > > + }
> > > > > +
> > > > > + subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte);
> > > >
> > > > I see that all pages passed in should be done after FOLL_SPLIT_PMD, so
> > > > is
> > > > this needed? Or say, should subpage==page always be true?
> > >
> > > Not always, in the case of a thp there are small ptes which will get
> > > device
> > > exclusive entries.
> >
> > FOLL_SPLIT_PMD will first split the huge thp into smaller pages, then do
> > follow_page_pte() on them (in follow_pmd_mask):
> >
> > if (flags & FOLL_SPLIT_PMD) {
> > int ret;
> > page = pmd_page(*pmd);
> > if (is_huge_zero_page(page)) {
> > spin_unlock(ptl);
> > ret = 0;
> > split_huge_pmd(vma, pmd, address);
> > if (pmd_trans_unstable(pmd))
> > ret = -EBUSY;
> > } else {
> > spin_unlock(ptl);
> > split_huge_pmd(vma, pmd, address);
> > ret = pte_alloc(mm, pmd) ? -ENOMEM : 0;
> > }
> >
> > return ret ? ERR_PTR(ret) :
> > follow_page_pte(vma, address, pmd, flags,
> > &ctx->pgmap); }
> >
> > So I thought all pages are small pages?
>
> The page will remain as a transparent huge page though (at least as I
> understand things). FOLL_SPLIT_PMD turns it into a pte mapped thp by splitting
> the pmd and creating pte's mapping the subpages but doesn't split the page
> itself. For comparison FOLL_SPLIT (which has been removed in v5.13 due to lack
> of use) is what would be used to split the page in the above GUP code by
> calling split_huge_page() rather than split_huge_pmd().
But shouldn't FOLL_SPLIT_PMD filled in small pfns for each pte? See
__split_huge_pmd_locked():
for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
...
} else {
entry = mk_pte(page + i, READ_ONCE(vma->vm_page_prot));
...
}
...
set_pte_at(mm, addr, pte, entry);
}
Then iiuc the coming follow_page_pte() will directly fetch the small pages?
--
Peter Xu
next prev parent reply other threads:[~2021-05-28 13:11 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-24 13:27 [PATCH v9 00/10] Add support for SVM atomics in Nouveau Alistair Popple
2021-05-24 13:27 ` [PATCH v9 01/10] mm: Remove special swap entry functions Alistair Popple
2021-05-24 13:27 ` [PATCH v9 02/10] mm/swapops: Rework swap entry manipulation code Alistair Popple
2021-05-24 13:27 ` [PATCH v9 03/10] mm/rmap: Split try_to_munlock from try_to_unmap Alistair Popple
2021-05-25 18:39 ` Liam Howlett
2021-05-25 23:45 ` Shakeel Butt
2021-06-04 20:49 ` Liam Howlett
2021-06-05 0:41 ` Shakeel Butt
2021-06-05 3:39 ` Liam Howlett
2021-06-05 4:19 ` Shakeel Butt
2021-06-07 4:51 ` Alistair Popple
2021-05-24 13:27 ` [PATCH v9 04/10] mm/rmap: Split migration into its own function Alistair Popple
2021-05-24 13:27 ` [PATCH v9 05/10] mm: Rename migrate_pgmap_owner Alistair Popple
2021-05-26 19:41 ` Peter Xu
2021-05-24 13:27 ` [PATCH v9 06/10] mm/memory.c: Allow different return codes for copy_nonpresent_pte() Alistair Popple
2021-05-26 19:50 ` Peter Xu
2021-05-27 1:20 ` Alistair Popple
2021-05-27 1:44 ` Peter Xu
2021-05-24 13:27 ` [PATCH v9 07/10] mm: Device exclusive memory access Alistair Popple
2021-05-24 22:11 ` Andrew Morton
2021-05-25 1:31 ` John Hubbard
2021-05-25 9:21 ` Alistair Popple
2021-05-25 11:51 ` Balbir Singh
2021-05-26 7:17 ` John Hubbard
2021-05-26 13:30 ` Alistair Popple
2021-06-02 8:50 ` Balbir Singh
2021-06-02 14:37 ` Peter Xu
2021-06-03 11:39 ` Alistair Popple
2021-06-03 14:47 ` Peter Xu
2021-06-04 1:07 ` Alistair Popple
2021-06-04 15:20 ` Peter Xu
2021-06-03 8:37 ` John Hubbard
2021-05-26 19:28 ` Peter Xu
2021-05-27 3:35 ` Alistair Popple
2021-05-27 13:04 ` Peter Xu
2021-05-28 1:48 ` Alistair Popple
2021-05-28 13:11 ` Peter Xu [this message]
2021-05-24 13:27 ` [PATCH v9 08/10] mm: Selftests for exclusive device memory Alistair Popple
2021-05-24 13:27 ` [PATCH v9 09/10] nouveau/svm: Refactor nouveau_range_fault Alistair Popple
2021-05-24 13:27 ` [PATCH v9 10/10] nouveau/svm: Implement atomic SVM access Alistair Popple
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