Re: [PATCH 08/12] gup: Don't allow FOLL_LONGTERM pinning of FS DAX pages

Alistair Popple
2024-10-14T07:12:56Z

Dan Williams <dan.j.williams@intel.com> writes:

> Dan Williams wrote:
>> Alistair Popple wrote:
>> > Longterm pinning of FS DAX pages should already be disallowed by
>> > various pXX_devmap checks. However a future change will cause these
>> > checks to be invalid for FS DAX pages so make
>> > folio_is_longterm_pinnable() return false for FS DAX pages.
>> >
>> > Signed-off-by: Alistair Popple <apopple@nvidia.com>
>> > ---
>> > include/linux/memremap.h | 11 +++++++++++
>> > include/linux/mm.h | 4 ++++
>> > 2 files changed, 15 insertions(+)
>> >
>> > diff --git a/include/linux/memremap.h b/include/linux/memremap.h
>> > index 14273e6..6a1406a 100644
>> > --- a/include/linux/memremap.h
>> > +++ b/include/linux/memremap.h
>> > @@ -187,6 +187,17 @@ static inline bool folio_is_device_coherent(const struct folio *folio)
>> > return is_device_coherent_page(&folio->page);
>> > }
>> >
>> > +static inline bool is_device_dax_page(const struct page *page)
>> > +{
>> > + return is_zone_device_page(page) &&
>> > + page_dev_pagemap(page)->type == MEMORY_DEVICE_FS_DAX;
>> > +}
>> > +
>> > +static inline bool folio_is_device_dax(const struct folio *folio)
>> > +{
>> > + return is_device_dax_page(&folio->page);
>> > +}
>> > +
>> > #ifdef CONFIG_ZONE_DEVICE
>> > void zone_device_page_init(struct page *page);
>> > void *memremap_pages(struct dev_pagemap *pgmap, int nid);
>> > diff --git a/include/linux/mm.h b/include/linux/mm.h
>> > index ae6d713..935e493 100644
>> > --- a/include/linux/mm.h
>> > +++ b/include/linux/mm.h
>> > @@ -1989,6 +1989,10 @@ static inline bool folio_is_longterm_pinnable(struct folio *folio)
>> > if (folio_is_device_coherent(folio))
>> > return false;
>> >
>> > + /* DAX must also always allow eviction. */
>> > + if (folio_is_device_dax(folio))
>>
>> Why is this called "folio_is_device_dax()" when the check is for fsdax?
>>
>> I would expect:
>>
>> if (folio_is_fsdax(folio))
>> return false;
>>
>> ...and s/device_dax/fsdax/ for the rest of the helpers.
>
> Specifically devdax is ok to allow longterm pinning since it is
> statically allocated. fsdax is the only ZONE_DEVICE mode where there is
> a higher-level allocator that does not support a 3rd party the block its
> operations indefinitely with a pin. So this needs to be explicit for
> that case.

Yeah, that all makes sense. I see what I did - was thinking in terms of
is this a zone device page - is_device - and if so what type
_(fs)dax. folio_is_fsdax() is much clearer though, thanks!