Re: [PATCH] mm: fix missing wake-up event for FSDAX pages

[Dan Williams <dan.j.williams@intel.com>]

Muchun Song wrote:
> On Mon, Jul 04, 2022 at 11:38:16AM +0100, Matthew Wilcox wrote:
> > On Mon, Jul 04, 2022 at 03:40:54PM +0800, Muchun Song wrote:
> > > FSDAX page refcounts are 1-based, rather than 0-based: if refcount is
> > > 1, then the page is freed.  The FSDAX pages can be pinned through GUP,
> > > then they will be unpinned via unpin_user_page() using a folio variant
> > > to put the page, however, folio variants did not consider this special
> > > case, the result will be to miss a wakeup event (like the user of
> > > __fuse_dax_break_layouts()).
> > 
> > Argh, no.  The 1-based refcounts are a blight on the entire kernel.
> > They need to go away, not be pushed into folios as well.  I think
> 
> I would be happy if this could go away.

Continue to agree that this blight needs to end.

One of the pre-requisites to getting back to normal accounting of FSDAX
page pin counts was to first drop the usage of get_dev_pagemap() in the
GUP path:

https://lore.kernel.org/linux-mm/161604048257.1463742.1374527716381197629.stgit@dwillia2-desk3.amr.corp.intel.com/

That work stalled on notifying mappers of surprise removal events of FSDAX pfns.
However, Ruan has been spending some time on that problem recently:

https://lore.kernel.org/all/20220410171623.3788004-1-ruansy.fnst@fujitsu.com/

So, once I dig out from a bit of CXL backlog and review that effort the
next step that I see will be convert the FSDAX path to take typical
references vmf_insert() time. Unless I am missing a shorter path to get
this fixed up?

> > we're close to having that fixed, but until then, this should do
> > the trick?
> > 
> 
> The following fix looks good to me since it lowers the overhead as
> much as possible

This change looks good to me as well.

> 
> Thanks.
> 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index cc98ab012a9b..4cef5e0f78b6 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1129,18 +1129,18 @@ static inline bool is_zone_movable_page(const struct page *page)
> >  #if defined(CONFIG_ZONE_DEVICE) && defined(CONFIG_FS_DAX)
> >  DECLARE_STATIC_KEY_FALSE(devmap_managed_key);
> >  
> > -bool __put_devmap_managed_page(struct page *page);
> > -static inline bool put_devmap_managed_page(struct page *page)
> > +bool __put_devmap_managed_page(struct page *page, int refs);
> > +static inline bool put_devmap_managed_page(struct page *page, int refs)
> >  {
> >  	if (!static_branch_unlikely(&devmap_managed_key))
> >  		return false;
> >  	if (!is_zone_device_page(page))
> >  		return false;
> > -	return __put_devmap_managed_page(page);
> > +	return __put_devmap_managed_page(page, refs);
> >  }
> >  
> >  #else /* CONFIG_ZONE_DEVICE && CONFIG_FS_DAX */
> > -static inline bool put_devmap_managed_page(struct page *page)
> > +static inline bool put_devmap_managed_page(struct page *page, int refs)
> >  {
> >  	return false;
> >  }
> > @@ -1246,7 +1246,7 @@ static inline void put_page(struct page *page)
> >  	 * For some devmap managed pages we need to catch refcount transition
> >  	 * from 2 to 1:
> >  	 */
> > -	if (put_devmap_managed_page(&folio->page))
> > +	if (put_devmap_managed_page(&folio->page, 1))
> >  		return;
> >  	folio_put(folio);
> >  }
> > diff --git a/mm/gup.c b/mm/gup.c
> > index d1132b39aa8f..28df02121c78 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -88,7 +88,8 @@ static inline struct folio *try_get_folio(struct page *page, int refs)
> >  	 * belongs to this folio.
> >  	 */
> >  	if (unlikely(page_folio(page) != folio)) {
> > -		folio_put_refs(folio, refs);
> > +		if (!put_devmap_managed_page(&folio->page, refs))
> > +			folio_put_refs(folio, refs);
> >  		goto retry;
> >  	}
> >  
> > @@ -177,6 +178,8 @@ static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
> >  			refs *= GUP_PIN_COUNTING_BIAS;
> >  	}
> >  
> > +	if (put_devmap_managed_page(&folio->page, refs))
> > +		return;
> >  	folio_put_refs(folio, refs);
> >  }
> >  
> > diff --git a/mm/memremap.c b/mm/memremap.c
> > index b870a659eee6..b25e40e3a11e 100644
> > --- a/mm/memremap.c
> > +++ b/mm/memremap.c
> > @@ -499,7 +499,7 @@ void free_zone_device_page(struct page *page)
> >  }
> >  
> >  #ifdef CONFIG_FS_DAX
> > -bool __put_devmap_managed_page(struct page *page)
> > +bool __put_devmap_managed_page(struct page *page, int refs)
> >  {
> >  	if (page->pgmap->type != MEMORY_DEVICE_FS_DAX)
> >  		return false;
> > @@ -509,7 +509,7 @@ bool __put_devmap_managed_page(struct page *page)
> >  	 * refcount is 1, then the page is free and the refcount is
> >  	 * stable because nobody holds a reference on the page.
> >  	 */
> > -	if (page_ref_dec_return(page) == 1)
> > +	if (page_ref_sub_return(page, refs) == 1)
> >  		wake_up_var(&page->_refcount);
> >  	return true;
> >  }
> > diff --git a/mm/swap.c b/mm/swap.c
> > index c6194cfa2af6..94e42a9bab92 100644
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -960,7 +960,7 @@ void release_pages(struct page **pages, int nr)
> >  				unlock_page_lruvec_irqrestore(lruvec, flags);
> >  				lruvec = NULL;
> >  			}
> > -			if (put_devmap_managed_page(&folio->page))
> > +			if (put_devmap_managed_page(&folio->page, 1))
> >  				continue;
> >  			if (folio_put_testzero(folio))
> >  				free_zone_device_page(&folio->page);
> >