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Message-ID: <359a1cf2-c5b0-4682-ba3c-980d77c4cfdb@redhat.com>
Date: Tue, 17 Dec 2024 23:14:42 +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 10/25] mm/mm_init: Move p2pdma page refcount
initialisation to p2pdma
On 17.12.24 06:12, Alistair Popple wrote:
> Currently ZONE_DEVICE page reference counts are initialised by core
> memory management code in __init_zone_device_page() as part of the
> memremap() call which driver modules make to obtain ZONE_DEVICE
> pages. This initialises page refcounts to 1 before returning them to
> the driver.
>
> This was presumably done because it drivers had a reference of sorts
> on the page. It also ensured the page could always be mapped with
> vm_insert_page() for example and would never get freed (ie. have a
> zero refcount), freeing drivers of manipulating page reference counts.
It probably dates back to copying that code from other zone-init code
where we
(a) Treat all available-at-boot memory as allocated before we release it
to the buddy
(b) Treat all hotplugged memory as allocated until we release it to the
buddy
As a side note, I'm working on converting (b) -- PageOffline pages -- to
have a refcount of 0 ("frozen").
>
> However it complicates figuring out whether or not a page is free from
> the mm perspective because it is no longer possible to just look at
> the refcount. Instead the page type must be known and if GUP is used a
> secondary pgmap reference is also sometimes needed.
>
> To simplify this it is desirable to remove the page reference count
> for the driver, so core mm can just use the refcount without having to
> account for page type or do other types of tracking. This is possible
> because drivers can always assume the page is valid as core kernel
> will never offline or remove the struct page.
>
> This means it is now up to drivers to initialise the page refcount as
> required. P2PDMA uses vm_insert_page() to map the page, and that
> requires a non-zero reference count when initialising the page so set
> that when the page is first mapped.
>
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> Reviewed-by: Dan Williams <dan.j.williams@...el.com>
>
> ---
>
> Changes since v2:
>
> - Initialise the page refcount for all pages covered by the kaddr
> ---
> drivers/pci/p2pdma.c | 13 +++++++++++--
> mm/memremap.c | 17 +++++++++++++----
> mm/mm_init.c | 22 ++++++++++++++++++----
> 3 files changed, 42 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 0cb7e0a..04773a8 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -140,13 +140,22 @@ static int p2pmem_alloc_mmap(struct file *filp, struct kobject *kobj,
> rcu_read_unlock();
>
> for (vaddr = vma->vm_start; vaddr < vma->vm_end; vaddr += PAGE_SIZE) {
> - ret = vm_insert_page(vma, vaddr, virt_to_page(kaddr));
> + struct page *page = virt_to_page(kaddr);
> +
> + /*
> + * Initialise the refcount for the freshly allocated page. As
> + * we have just allocated the page no one else should be
> + * using it.
> + */
> + VM_WARN_ON_ONCE_PAGE(!page_ref_count(page), page);
> + set_page_count(page, 1);
> + ret = vm_insert_page(vma, vaddr, page);
> if (ret) {
> gen_pool_free(p2pdma->pool, (uintptr_t)kaddr, len);
> return ret;
> }
> percpu_ref_get(ref);
> - put_page(virt_to_page(kaddr));
> + put_page(page);
> kaddr += PAGE_SIZE;
> len -= PAGE_SIZE;
> }
> diff --git a/mm/memremap.c b/mm/memremap.c
> index 40d4547..07bbe0e 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -488,15 +488,24 @@ void free_zone_device_folio(struct folio *folio)
> folio->mapping = NULL;
> folio->page.pgmap->ops->page_free(folio_page(folio, 0));
>
> - if (folio->page.pgmap->type != MEMORY_DEVICE_PRIVATE &&
> - folio->page.pgmap->type != MEMORY_DEVICE_COHERENT)
> + switch (folio->page.pgmap->type) {
> + case MEMORY_DEVICE_PRIVATE:
> + case MEMORY_DEVICE_COHERENT:
> + put_dev_pagemap(folio->page.pgmap);
> + break;
> +
> + case MEMORY_DEVICE_FS_DAX:
> + case MEMORY_DEVICE_GENERIC:
> /*
> * Reset the refcount to 1 to prepare for handing out the page
> * again.
> */
> folio_set_count(folio, 1);
> - else
> - put_dev_pagemap(folio->page.pgmap);
> + break;
> +
> + case MEMORY_DEVICE_PCI_P2PDMA:
> + break;
> + }
> }
>
> void zone_device_page_init(struct page *page)
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 24b68b4..f021e63 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1017,12 +1017,26 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
> }
>
> /*
> - * ZONE_DEVICE pages are released directly to the driver page allocator
> - * which will set the page count to 1 when allocating the page.
> + * ZONE_DEVICE pages other than MEMORY_TYPE_GENERIC and
> + * MEMORY_TYPE_FS_DAX pages are released directly to the driver page
> + * allocator which will set the page count to 1 when allocating the
> + * page.
> + *
> + * MEMORY_TYPE_GENERIC and MEMORY_TYPE_FS_DAX pages automatically have
> + * their refcount reset to one whenever they are freed (ie. after
> + * their refcount drops to 0).
> */
> - if (pgmap->type == MEMORY_DEVICE_PRIVATE ||
> - pgmap->type == MEMORY_DEVICE_COHERENT)
> + switch (pgmap->type) {
> + case MEMORY_DEVICE_PRIVATE:
> + case MEMORY_DEVICE_COHERENT:
> + case MEMORY_DEVICE_PCI_P2PDMA:
> set_page_count(page, 0);
> + break;
> +
> + case MEMORY_DEVICE_FS_DAX:
> + case MEMORY_DEVICE_GENERIC:
> + break;
> + }
> }
>
> /*
But that's a bit weird: we call __init_single_page()->init_page_count()
to initialize it to 1, to then set it back to 0.
Maybe we can just pass to __init_single_page() the refcount we want to
have directly? Can be a patch on top of course.
Apart from that
Acked-by: David Hildenbrand <david@...hat.com>
--
Cheers,
David / dhildenb
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