Re: [PATCH v2 5/5] mm: Add ZAP_FLAG_SKIP_SWAP and zap_flags

[Peter Xu]

On Fri, Sep 03, 2021 at 09:25:51AM +0200, David Hildenbrand wrote:
> On 02.09.21 22:18, 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
> 
> s/on//
> 
> > 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.
> 
> s/errornously/erroneously/

Will fix, thanks.

> 
> > 
> > Actually very recently we introduced unmap_mapping_page() in 22061a1ffabd, I
> > think that should also look into swap entries.  Add a comment there.  IOW, this
> > patch will be a functional change to unmap_mapping_page() but hopefully in the
> > right way to do it.

I'll remove this paragraph, as explained elsewhere.

> > 
> > 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 should still
> > be the major case here).
> > 
> > Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
> > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > ---
> >   include/linux/mm.h | 16 ++++++++++++++++
> >   mm/memory.c        |  6 +++---
> >   2 files changed, 19 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 81e402a5fbc9..a7bcdb2ec956 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1716,12 +1716,18 @@ static inline bool can_do_mlock(void) { return false; }
> >   extern int user_shm_lock(size_t, struct ucounts *);
> >   extern void user_shm_unlock(size_t, struct ucounts *);
> > +typedef unsigned int __bitwise zap_flags_t;
> > +
> > +/* Whether to skip zapping swap entries */
> > +#define  ZAP_FLAG_SKIP_SWAP  ((__force zap_flags_t) BIT(0))
> 
> Interestingly, this will also skip fake some swap entries (e.g., migration
> entries but not private/exclusive entries). Maybe extend that documentation
> a bit.
> 
> ... but, looking into zap_pmd_range(), we don't care about "details" when
> calling zap_huge_pmd(), which will zap pmd migration entries IIUC ... so
> it's really unclear to me what the flag (and current behavior) really is and
> what should be documented. Should we maybe really only care about "real"
> swap entries?

Indeed, I tried to look into it today and see why we wanted to skip swap
entries but I failed to figure it out easily - it goes back to the 1st git
commit of Linux.

Maybe there'll be some experienced developer who knows the history, but before
that I decided to just keep the behavior.

The final goal of mine is to make the code clean and also prepares for the
uffd-wp that will allow free-style use of "details" pointer, rather than have
an implicit hint on "skip swap entry" then it'll be enough for this patch.

> 
> Most probably I'm just missing something important.
> 
> > +
> >   /*
> >    * Parameter block passed down to zap_pte_range in exceptional cases.
> >    */
> >   struct zap_details {
> >   	struct address_space *zap_mapping;	/* Check page->mapping if set */
> >   	struct page *single_page;		/* Locked page to be unmapped */
> > +	zap_flags_t zap_flags;			/* Extra flags for zapping */
> >   };
> >   /*
> > @@ -1737,6 +1743,16 @@ zap_skip_check_mapping(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)
> > +{
> > +	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 e5ee8399d270..4cb269ca8249 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -1379,8 +1379,7 @@ 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))
> > @@ -3351,6 +3350,7 @@ void unmap_mapping_page(struct page *page)
> >   	first_index = page->index;
> >   	last_index = page->index + thp_nr_pages(page) - 1;
> > +	/* Keep ZAP_FLAG_SKIP_SWAP cleared because we're truncating */

As to keep the behavior, I shouldn't fiddle around with this, so I'll also
attach ZAP_FLAG_SKIP_SWAP to unmap_mapping_page() too in v3.

> >   	details.zap_mapping = mapping;
> >   	details.single_page = page;

Thanks,

-- 
Peter Xu