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Message-ID: <4b5768b7-96e0-4864-9dbe-88fd1f0e87b8@redhat.com>
Date: Tue, 17 Dec 2024 23:27:13 +0100
From: David Hildenbrand <david@...hat.com>
To: Alistair Popple <apopple@...dia.com>, akpm@...ux-foundation.org,
dan.j.williams@...el.com, linux-mm@...ck.org
Cc: lina@...hilina.net, zhang.lyra@...il.com, gerald.schaefer@...ux.ibm.com,
vishal.l.verma@...el.com, dave.jiang@...el.com, logang@...tatee.com,
bhelgaas@...gle.com, jack@...e.cz, jgg@...pe.ca, catalin.marinas@....com,
will@...nel.org, mpe@...erman.id.au, npiggin@...il.com,
dave.hansen@...ux.intel.com, ira.weiny@...el.com, willy@...radead.org,
djwong@...nel.org, tytso@....edu, linmiaohe@...wei.com, peterx@...hat.com,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
nvdimm@...ts.linux.dev, linux-cxl@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
linux-xfs@...r.kernel.org, jhubbard@...dia.com, hch@....de,
david@...morbit.com
Subject: Re: [PATCH v4 14/25] rmap: Add support for PUD sized mappings to rmap
On 17.12.24 06:12, Alistair Popple wrote:
> The rmap doesn't currently support adding a PUD mapping of a
> folio. This patch adds support for entire PUD mappings of folios,
> primarily to allow for more standard refcounting of device DAX
> folios. Currently DAX is the only user of this and it doesn't require
> support for partially mapped PUD-sized folios so we don't support for
> that for now.
>
> Signed-off-by: Alistair Popple <apopple@...dia.com>
>
> ---
>
> David - Thanks for your previous comments, I'm less familiar with the
> rmap code so I would appreciate you taking another look. In particular
> I haven't added a stat for PUD mapped folios as it seemed like
> overkill for just the device DAX case but let me know if you think
> otherwise.
>
> Changes for v4:
>
> - New for v4, split out rmap changes as suggested by David.
> ---
> include/linux/rmap.h | 15 ++++++++++++-
> mm/rmap.c | 56 +++++++++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 71 insertions(+)
>
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index 683a040..7043914 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -192,6 +192,7 @@ typedef int __bitwise rmap_t;
> enum rmap_level {
> RMAP_LEVEL_PTE = 0,
> RMAP_LEVEL_PMD,
> + RMAP_LEVEL_PUD,
> };
>
> static inline void __folio_rmap_sanity_checks(const struct folio *folio,
> @@ -228,6 +229,14 @@ static inline void __folio_rmap_sanity_checks(const struct folio *folio,
> VM_WARN_ON_FOLIO(folio_nr_pages(folio) != HPAGE_PMD_NR, folio);
> VM_WARN_ON_FOLIO(nr_pages != HPAGE_PMD_NR, folio);
> break;
> + case RMAP_LEVEL_PUD:
> + /*
> + * Assume that we are creating * a single "entire" mapping of the
> + * folio.
> + */
> + VM_WARN_ON_FOLIO(folio_nr_pages(folio) != HPAGE_PUD_NR, folio);
> + VM_WARN_ON_FOLIO(nr_pages != HPAGE_PUD_NR, folio);
> + break;
> default:
> VM_WARN_ON_ONCE(true);
> }
> @@ -251,12 +260,16 @@ void folio_add_file_rmap_ptes(struct folio *, struct page *, int nr_pages,
> folio_add_file_rmap_ptes(folio, page, 1, vma)
> void folio_add_file_rmap_pmd(struct folio *, struct page *,
> struct vm_area_struct *);
> +void folio_add_file_rmap_pud(struct folio *, struct page *,
> + struct vm_area_struct *);
> void folio_remove_rmap_ptes(struct folio *, struct page *, int nr_pages,
> struct vm_area_struct *);
> #define folio_remove_rmap_pte(folio, page, vma) \
> folio_remove_rmap_ptes(folio, page, 1, vma)
> void folio_remove_rmap_pmd(struct folio *, struct page *,
> struct vm_area_struct *);
> +void folio_remove_rmap_pud(struct folio *, struct page *,
> + struct vm_area_struct *);
>
> void hugetlb_add_anon_rmap(struct folio *, struct vm_area_struct *,
> unsigned long address, rmap_t flags);
> @@ -341,6 +354,7 @@ static __always_inline void __folio_dup_file_rmap(struct folio *folio,
> atomic_add(orig_nr_pages, &folio->_large_mapcount);
> break;
> case RMAP_LEVEL_PMD:
> + case RMAP_LEVEL_PUD:
> atomic_inc(&folio->_entire_mapcount);
> atomic_inc(&folio->_large_mapcount);
> break;
> @@ -437,6 +451,7 @@ static __always_inline int __folio_try_dup_anon_rmap(struct folio *folio,
> atomic_add(orig_nr_pages, &folio->_large_mapcount);
> break;
> case RMAP_LEVEL_PMD:
> + case RMAP_LEVEL_PUD:
> if (PageAnonExclusive(page)) {
> if (unlikely(maybe_pinned))
> return -EBUSY;
> diff --git a/mm/rmap.c b/mm/rmap.c
> index c6c4d4e..39d0439 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1203,6 +1203,11 @@ static __always_inline unsigned int __folio_add_rmap(struct folio *folio,
> }
> atomic_inc(&folio->_large_mapcount);
> break;
> + case RMAP_LEVEL_PUD:
> + /* We only support entire mappings of PUD sized folios in rmap */
> + atomic_inc(&folio->_entire_mapcount);
> + atomic_inc(&folio->_large_mapcount);
> + break;
This way you don't account the pages at all as mapped, whereby PTE-mapping it
would? And IIRC, these PUD-sized pages can be either mapped using PTEs or
using a single PUD.
I suspect what you want is to
diff --git a/mm/rmap.c b/mm/rmap.c
index c6c4d4ea29a7e..1477028d3a176 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1187,12 +1187,19 @@ static __always_inline unsigned int __folio_add_rmap(struct folio *folio,
atomic_add(orig_nr_pages, &folio->_large_mapcount);
break;
case RMAP_LEVEL_PMD:
+ case RMAP_LEVEL_PUD:
first = atomic_inc_and_test(&folio->_entire_mapcount);
if (first) {
nr = atomic_add_return_relaxed(ENTIRELY_MAPPED, mapped);
if (likely(nr < ENTIRELY_MAPPED + ENTIRELY_MAPPED)) {
- *nr_pmdmapped = folio_nr_pages(folio);
- nr = *nr_pmdmapped - (nr & FOLIO_PAGES_MAPPED);
+ nr_pages = folio_nr_pages(folio);
+ /*
+ * We only track PMD mappings of PMD-sized
+ * folios separately.
+ */
+ if (level == RMAP_LEVEL_PMD)
+ *nr_pmdmapped = nr_pages;
+ nr = nr_pages - (nr & FOLIO_PAGES_MAPPED);
/* Raced ahead of a remove and another add? */
if (unlikely(nr < 0))
nr = 0;
Similar on the removal path.
--
Cheers,
David / dhildenb
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