Re: [PATCH v3 08/27] mm: Introduce zap_details.zap_flags

[Peter Xu <peterx@redhat.com>]

On Mon, Jun 21, 2021 at 10:09:00PM +1000, Alistair Popple wrote:
> On Friday, 28 May 2021 6:21:30 AM AEST Peter Xu wrote:
> > Instead of trying to introduce one variable for every new zap_details fields,
> > let's introduce a flag so that it can start to encode true/false informations.
> > 
> > Let's start to use this flag first to clean up the only check_mapping variable.
> > Firstly, the name "check_mapping" implies this is a "boolean", but actually it
> > stores the mapping inside, just in a way that it won't be set if we don't want
> > to check the mapping.
> > 
> > To make things clearer, introduce the 1st zap flag ZAP_FLAG_CHECK_MAPPING, so
> > that we only check against the mapping if this bit set.  At the same time, we
> > can rename check_mapping into zap_mapping and set it always.
> > 
> > Since at it, introduce another helper zap_check_mapping_skip() and use it in
> > zap_pte_range() properly.
> > 
> > Some old comments have been removed in zap_pte_range() because they're
> > duplicated, and since now we're with ZAP_FLAG_CHECK_MAPPING flag, it'll be very
> > easy to grep this information by simply grepping the flag.
> > 
> > It'll also make life easier when we want to e.g. pass in zap_flags into the
> > callers like unmap_mapping_pages() (instead of adding new booleans besides the
> > even_cows parameter).
> > 
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >  include/linux/mm.h | 19 ++++++++++++++++++-
> >  mm/memory.c        | 31 ++++++++-----------------------
> >  2 files changed, 26 insertions(+), 24 deletions(-)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index db155be8e66c..52d3ef2ed753 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1721,13 +1721,30 @@ static inline bool can_do_mlock(void) { return false; }
> >  extern int user_shm_lock(size_t, struct user_struct *);
> >  extern void user_shm_unlock(size_t, struct user_struct *);
> >  
> > +/* Whether to check page->mapping when zapping */
> > +#define  ZAP_FLAG_CHECK_MAPPING             BIT(0)
> > +
> >  /*
> >   * Parameter block passed down to zap_pte_range in exceptional cases.
> >   */
> >  struct zap_details {
> > -	struct address_space *check_mapping;	/* Check page->mapping if set */
> > +	struct address_space *zap_mapping;
> > +	unsigned long zap_flags;
> >  };
> >  
> > +/* Return true if skip zapping this page, false otherwise */
> > +static inline bool
> > +zap_check_mapping_skip(struct zap_details *details, struct page *page)
> > +{
> > +	if (!details || !page)
> > +		return false;
> > +
> > +	if (!(details->zap_flags & ZAP_FLAG_CHECK_MAPPING))
> > +		return false;

[1]

> > +
> > +	return details->zap_mapping != page_rmapping(page);
> 
> I doubt this matters in practice, but there is a slight behaviour change
> here that might be worth checking. Previously this check was equivalent
> to:
> 
> details->zap_mapping && details->zap_mapping != page_rmapping(page)

Yes; IMHO "details->zap_mapping" is just replaced by the check at [1].

For example, there's only one real user of this mapping check, which is
unmap_mapping_pages() below [2].

With the old code, we have:

    details.check_mapping = even_cows ? NULL : mapping;

So "details->zap_mapping" is only true if "!even_cows".

With the new code, we'll have:

    if (!even_cows)
        details.zap_flags |= ZAP_FLAG_CHECK_MAPPING;

So ZAP_FLAG_CHECK_MAPPING is only set if "!even_cows", while that's what we
check exactly at [1].

> 
> Otherwise I think this looks good.
> 
> > +}
> > +
> >  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 27cf8a6375c6..c9dc4e9e05b5 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -1330,16 +1330,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
> >  			struct page *page;
> >  
> >  			page = vm_normal_page(vma, addr, ptent);
> > -			if (unlikely(details) && page) {
> > -				/*
> > -				 * unmap_shared_mapping_pages() wants to
> > -				 * invalidate cache without truncating:
> > -				 * unmap shared but keep private pages.
> > -				 */
> > -				if (details->check_mapping &&
> > -				    details->check_mapping != page_rmapping(page))
> > -					continue;
> > -			}
> > +			if (unlikely(zap_check_mapping_skip(details, page)))
> > +				continue;
> >  			ptent = ptep_get_and_clear_full(mm, addr, pte,
> >  							tlb->fullmm);
> >  			tlb_remove_tlb_entry(tlb, pte, addr);
> > @@ -1372,17 +1364,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
> >  		    is_device_exclusive_entry(entry)) {
> >  			struct page *page = pfn_swap_entry_to_page(entry);
> >  
> > -			if (unlikely(details && details->check_mapping)) {
> > -				/*
> > -				 * unmap_shared_mapping_pages() wants to
> > -				 * invalidate cache without truncating:
> > -				 * unmap shared but keep private pages.
> > -				 */
> > -				if (details->check_mapping !=
> > -				    page_rmapping(page))
> > -					continue;
> > -			}
> > -
> > +			if (unlikely(zap_check_mapping_skip(details, page)))
> > +				continue;
> >  			pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
> >  			rss[mm_counter(page)]--;
> >  
> > @@ -3345,9 +3328,11 @@ 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 = { };
> > +	struct zap_details details = { .zap_mapping = mapping };
> > +
> > +	if (!even_cows)
> > +		details.zap_flags |= ZAP_FLAG_CHECK_MAPPING;
> >  
> > -	details.check_mapping = even_cows ? NULL : mapping;

[2]

> >  	if (last_index < first_index)
> >  		last_index = ULONG_MAX;

Thanks,

-- 
Peter Xu