Re: [v3 PATCH 1/2] mm: rmap: make try_to_unmap() void function

[Hugh Dickins]

On Tue, 25 May 2021, Yang Shi wrote:

> Currently try_to_unmap() return bool value by checking page_mapcount(),
> however this may return false positive since page_mapcount() doesn't
> check all subpages of compound page.  The total_mapcount() could be used
> instead, but its cost is higher since it traverses all subpages.
> 
> Actually the most callers of try_to_unmap() don't care about the
> return value at all.  So just need check if page is still mapped by
> page_mapped() when necessary.  And page_mapped() does bail out early
> when it finds mapped subpage.
> 
> Suggested-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>

Thanks for doing this, I like it, you can add
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>

Let's all ignore checkpatch's complaint on "struct page *":
that try_to_unmap() prototype is consistent with others in rmap.h,
and changing them to "struct page *page" adds no information at all.

But a nit in mm/memory-failure.c: unmap_success no longer needs
to be initialized to true on entry to hwpoison_user_mappings().

More seriously, I question the ordering of the two patches:
I think this 1/2 should be 2/2, and the other be 1/2.  The other is
the one which replaces a BUG by a WARN, which will be wanted Cc stable;
whereas this one is really just a cleanup - I don't think there's a
serious consequence from the faint possibility of wrong unmap_success
on THP, in either memory-failure.c or vmscan.c - wrong message, wrong
stats, but more than that? And memory-failure.c on THP somewhat a
work in progress IIUC.

Unfortunately, they can't just be swapped but need rediffing.
Or do you disagree that's a better ordering?

As it stands, I imagine my series of THP unmap fixes (most needing Cc
stable) coming on top of your "mm: thp: check page_mapped instead of
page_mapcount for split" (umm, that's a bad title), but before this
cleanup.

Though I'm responding before I've looked through the stable trees:
it's not of overriding importance, but I'll want to try to minimize
the number of patches that need redoing for older stables, which might
affect the ordering.  (And it might end up easiest if I bring your two
patches into my series, then ask Andrew to replace.)

Further complicated by Andrew having brought in the nouveau reorg of
mm/rmap.c (oh, and that touches mm/huge_memory.c unmap_page() too).
I expect I'll have to send Andrew a series to go before all that,
together with fixups to that to sit on top of it.  Sigh.  Well,
we can only blame me for not getting those THP fixes in sooner.

Hugh

> ---
>  include/linux/rmap.h |  2 +-
>  mm/huge_memory.c     |  4 +---
>  mm/memory-failure.c  | 13 ++++++-------
>  mm/rmap.c            |  6 +-----
>  mm/vmscan.c          |  3 ++-
>  5 files changed, 11 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index def5c62c93b3..116cb193110a 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -194,7 +194,7 @@ static inline void page_dup_rmap(struct page *page, bool compound)
>  int page_referenced(struct page *, int is_locked,
>  			struct mem_cgroup *memcg, unsigned long *vm_flags);
>  
> -bool try_to_unmap(struct page *, enum ttu_flags flags);
> +void try_to_unmap(struct page *, enum ttu_flags flags);
>  
>  /* Avoid racy checks */
>  #define PVMW_SYNC		(1 << 0)
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 19195fca1aee..80fe642d742d 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2336,15 +2336,13 @@ static void unmap_page(struct page *page)
>  {
>  	enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK |
>  		TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
> -	bool unmap_success;
>  
>  	VM_BUG_ON_PAGE(!PageHead(page), page);
>  
>  	if (PageAnon(page))
>  		ttu_flags |= TTU_SPLIT_FREEZE;
>  
> -	unmap_success = try_to_unmap(page, ttu_flags);
> -	VM_BUG_ON_PAGE(!unmap_success, page);
> +	try_to_unmap(page, ttu_flags);
>  }
>  
>  static void remap_page(struct page *page, unsigned int nr)
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 9dcc9bcea731..6dd53ff34825 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1126,7 +1126,7 @@ static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
>  		collect_procs(hpage, &tokill, flags & MF_ACTION_REQUIRED);
>  
>  	if (!PageHuge(hpage)) {
> -		unmap_success = try_to_unmap(hpage, ttu);
> +		try_to_unmap(hpage, ttu);
>  	} else {
>  		if (!PageAnon(hpage)) {
>  			/*
> @@ -1138,17 +1138,16 @@ static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
>  			 */
>  			mapping = hugetlb_page_mapping_lock_write(hpage);
>  			if (mapping) {
> -				unmap_success = try_to_unmap(hpage,
> -						     ttu|TTU_RMAP_LOCKED);
> +				try_to_unmap(hpage, ttu|TTU_RMAP_LOCKED);
>  				i_mmap_unlock_write(mapping);
> -			} else {
> +			} else
>  				pr_info("Memory failure: %#lx: could not lock mapping for mapped huge page\n", pfn);
> -				unmap_success = false;
> -			}
>  		} else {
> -			unmap_success = try_to_unmap(hpage, ttu);
> +			try_to_unmap(hpage, ttu);
>  		}
>  	}
> +
> +	unmap_success = !page_mapped(hpage);
>  	if (!unmap_success)
>  		pr_err("Memory failure: %#lx: failed to unmap page (mapcount=%d)\n",
>  		       pfn, page_mapcount(hpage));
> diff --git a/mm/rmap.c b/mm/rmap.c
> index a35cbbbded0d..728de421e43a 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1748,10 +1748,8 @@ static int page_not_mapped(struct page *page)
>   *
>   * Tries to remove all the page table entries which are mapping this
>   * page, used in the pageout path.  Caller must hold the page lock.
> - *
> - * If unmap is successful, return true. Otherwise, false.
>   */
> -bool try_to_unmap(struct page *page, enum ttu_flags flags)
> +void try_to_unmap(struct page *page, enum ttu_flags flags)
>  {
>  	struct rmap_walk_control rwc = {
>  		.rmap_one = try_to_unmap_one,
> @@ -1776,8 +1774,6 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
>  		rmap_walk_locked(page, &rwc);
>  	else
>  		rmap_walk(page, &rwc);
> -
> -	return !page_mapcount(page) ? true : false;
>  }
>  
>  /**
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index f96d62159720..fa5052ace415 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1499,7 +1499,8 @@ static unsigned int shrink_page_list(struct list_head *page_list,
>  			if (unlikely(PageTransHuge(page)))
>  				flags |= TTU_SPLIT_HUGE_PMD;
>  
> -			if (!try_to_unmap(page, flags)) {
> +			try_to_unmap(page, flags);
> +			if (page_mapped(page)) {
>  				stat->nr_unmap_fail += nr_pages;
>  				if (!was_swapbacked && PageSwapBacked(page))
>  					stat->nr_lazyfree_fail += nr_pages;
> -- 
> 2.26.2