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

From: Alistair Popple <[email-masked]>
Date: Mon, 14 Oct 2024 18:03:17 +1100

Dan Williams <[email-masked]> 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 <[email-masked]>
>> > ---
>> >  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!