From: Matthew Wilcox <willy@infradead.org>
To: Liam Howlett <liam.howlett@oracle.com>
Cc: Alex Sierra <alex.sierra@amd.com>,
"rcampbell@nvidia.com" <rcampbell@nvidia.com>,
"Felix.Kuehling@amd.com" <Felix.Kuehling@amd.com>,
"amd-gfx@lists.freedesktop.org" <amd-gfx@lists.freedesktop.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"jglisse@redhat.com" <jglisse@redhat.com>,
"dri-devel@lists.freedesktop.org"
<dri-devel@lists.freedesktop.org>,
"jgg@nvidia.com" <jgg@nvidia.com>,
"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
"hch@lst.de" <hch@lst.de>
Subject: Re: [RFC PATCH v2 1/8] ext4/xfs: add page refcount helper
Date: Tue, 8 Jun 2021 15:33:05 +0100 [thread overview]
Message-ID: <YL9/ob6taZXHow5t@casper.infradead.org> (raw)
In-Reply-To: <20210608002858.gegg6sl2z2dwwstb@revolver>
On Tue, Jun 08, 2021 at 12:29:04AM +0000, Liam Howlett wrote:
> * Alex Sierra <alex.sierra@amd.com> [210607 16:43]:
> > From: Ralph Campbell <rcampbell@nvidia.com>
> >
> > There are several places where ZONE_DEVICE struct pages assume a reference
> > count == 1 means the page is idle and free. Instead of open coding this,
> > add a helper function to hide this detail.
> >
> > diff --git a/include/linux/dax.h b/include/linux/dax.h
> > index b52f084aa643..8909a91cd381 100644
> > --- a/include/linux/dax.h
> > +++ b/include/linux/dax.h
> > @@ -243,6 +243,16 @@ static inline bool dax_mapping(struct address_space *mapping)
> > return mapping->host && IS_DAX(mapping->host);
> > }
> >
> > +static inline bool dax_layout_is_idle_page(struct page *page)
> > +{
> > + return page_ref_count(page) == 1;
> > +}
>
> If this races with page_ref_count(page) == 0, then it will return false
> that a page is idle when the page is being freed. I don't know the code
> well enough to say if this is an issue or not so please let me know.
>
> For example:
> !dax_layout_is_idle_page() will return true in dax_busy_page() above
> when the count is 0 and return the page.
>
> Maybe you are sure to have at least one reference when calling this? It
> might be worth adding a comment.
You're getting confused by the problem that the next patch fixes, which
is that devmap pages were stupidly given an elevated refcount. devmap
pages are considered "free" when their refcount is 1. See
put_page(), put_devmap_managed_page() and so on.
next prev parent reply other threads:[~2021-06-08 14:33 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-07 20:42 [RFC PATCH v2 0/8] Support DEVICE_GENERIC memory in migrate_vma_* Alex Sierra
2021-06-07 20:42 ` [RFC PATCH v2 1/8] ext4/xfs: add page refcount helper Alex Sierra
2021-06-08 0:29 ` Liam Howlett
2021-06-08 14:33 ` Matthew Wilcox [this message]
2021-06-09 19:23 ` Matthew Wilcox
2021-06-14 15:26 ` Felix Kuehling
2021-06-07 20:42 ` [RFC PATCH v2 2/8] mm: remove extra ZONE_DEVICE struct page refcount Alex Sierra
2021-06-07 20:42 ` [RFC PATCH v2 3/8] kernel: resource: lookup_resource as exported symbol Alex Sierra
2021-06-07 20:42 ` [RFC PATCH v2 4/8] drm/amdkfd: add SPM support for SVM Alex Sierra
2021-06-07 20:42 ` [RFC PATCH v2 5/8] drm/amdkfd: generic type as sys mem on migration to ram Alex Sierra
2021-06-07 20:42 ` [RFC PATCH v2 6/8] include/linux/mm.h: helpers to check zone device generic type Alex Sierra
2021-06-07 20:42 ` [RFC PATCH v2 7/8] mm: add generic type support to migrate_vma helpers Alex Sierra
2021-06-07 20:42 ` [RFC PATCH v2 8/8] mm: call pgmap->ops->page_free for DEVICE_GENERIC pages Alex Sierra
2021-06-08 14:26 ` [RFC PATCH v2 0/8] Support DEVICE_GENERIC memory in migrate_vma_* Matthew Wilcox
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