Re: [PATCH v3 09/27] mm: Introduce ZAP_FLAG_SKIP_SWAP

[Alistair Popple]

On Tuesday, 22 June 2021 2:26:23 AM AEST Peter Xu wrote:
> On Mon, Jun 21, 2021 at 10:36:46PM +1000, Alistair Popple wrote:
> > On Friday, 28 May 2021 6:21:35 AM AEST Peter Xu wrote:
> > > Firstly, the comment in zap_pte_range() is misleading because it checks against
> > > details rather than check_mappings, so it's against what the code did.
> > > 
> > > Meanwhile, it's confusing too on not explaining why passing in the details
> > > pointer would mean to skip all swap entries.  New user of zap_details could
> > > very possibly miss this fact if they don't read deep until zap_pte_range()
> > > because there's no comment at zap_details talking about it at all, so swap
> > > entries could be errornously skipped without being noticed.
> > > 
> > > This partly reverts 3e8715fdc03e ("mm: drop zap_details::check_swap_entries"),
> > > but introduce ZAP_FLAG_SKIP_SWAP flag, which means the opposite of previous
> > > "details" parameter: the caller should explicitly set this to skip swap
> > > entries, otherwise swap entries will always be considered (which is still the
> > > major case here).
> > > 
> > > Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
> > > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > > ---
> > >  include/linux/mm.h | 12 ++++++++++++
> > >  mm/memory.c        |  8 +++++---
> > >  2 files changed, 17 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > index 52d3ef2ed753..1adf313a01fe 100644
> > > --- a/include/linux/mm.h
> > > +++ b/include/linux/mm.h
> > > @@ -1723,6 +1723,8 @@ extern void user_shm_unlock(size_t, struct user_struct *);
> > >  
> > >  /* Whether to check page->mapping when zapping */
> > >  #define  ZAP_FLAG_CHECK_MAPPING             BIT(0)
> > > +/* Whether to skip zapping swap entries */
> > > +#define  ZAP_FLAG_SKIP_SWAP                 BIT(1)
> > >  
> > >  /*
> > >   * Parameter block passed down to zap_pte_range in exceptional cases.
> > > @@ -1745,6 +1747,16 @@ zap_check_mapping_skip(struct zap_details *details, struct page *page)
> > >  	return details->zap_mapping != page_rmapping(page);
> > >  }
> > >  
> > > +/* Return true if skip swap entries, false otherwise */
> > > +static inline bool
> > > +zap_skip_swap(struct zap_details *details)
> > 
> > Minor nit-pick but imho it would be nice if the naming was consistent between
> > this and check mapping. Ie. zap_skip_swap()/zap_skip_check_mapping() or
> > zap_swap_skip()/zap_check_mapping_skip().
> 
> Makes sense; I'll use zap_skip_swap()/zap_skip_check_mapping() I think, then I
> keep this patch untouched.

Sounds good.

> > 
> > > +{
> > > +	if (!details)
> > > +		return false;
> > > +
> > > +	return details->zap_flags & ZAP_FLAG_SKIP_SWAP;
> > > +}
> > > +
> > >  struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
> > >  			     pte_t pte);
> > >  struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index c9dc4e9e05b5..8a3751be87ba 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -1376,8 +1376,7 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
> > >  			continue;
> > >  		}
> > >  
> > > -		/* If details->check_mapping, we leave swap entries. */
> > > -		if (unlikely(details))
> > > +		if (unlikely(zap_skip_swap(details)))
> > >  			continue;
> > >  
> > >  		if (!non_swap_entry(entry))
> > > @@ -3328,7 +3327,10 @@ void unmap_mapping_pages(struct address_space *mapping, pgoff_t start,
> > >  		pgoff_t nr, bool even_cows)
> > >  {
> > >  	pgoff_t	first_index = start, last_index = start + nr - 1;
> > > -	struct zap_details details = { .zap_mapping = mapping };
> > > +	struct zap_details details = {
> > > +		.zap_mapping = mapping,
> > 
> > I meant to comment on this in the previous patch, but it might be nice to set
> > .zap_mapping in the !even_cows case below to make it very obvious it only
> > applies to ZAP_FLAG_CHECK_MAPPING.
> 
> I wanted to make it easy to understand by having zap_mapping always points to
> the mapping it's zapping, so it does not contain any other information like
> "whether we want to check the mapping is the same when zap", which now stays
> fully in the flags. Then it's always legal to reference zap_mapping without any
> prior knowledge.  But indeed it's only used by ZAP_FLAG_CHECK_MAPPING.
> 
> I do have a slight preference to keep it as the patch does, but I don't have a
> strong opinion.  Let me know if you insist; I can change.

No insistence from me if you want to keep it this way, it's all pretty obvious
anyway.

> > 
> > Otherwise I think this is a good clean up which makes things clearer. I double
> > checked that unmap_mapping_pages() was the only place in the existing code that
> > needs ZAP_FLAG_SKIP_SWAP and that appears to be the case so there shouldn't be
> > any behaviour changes from this.
> > 
> > Reviewed-by: Alistair Popple <apopple@xxxxxxxxxx>
> 
> Since I won't change anything within this patch, I'll take this away, thanks!
> 
>