Re: [PATCH v9 03/10] mm/rmap: Split try_to_munlock from try_to_unmap

[Liam Howlett]

* Alistair Popple <apopple@xxxxxxxxxx> [210524 09:29]:
> The behaviour of try_to_unmap_one() is difficult to follow because it
> performs different operations based on a fairly large set of flags used
> in different combinations.
> 
> TTU_MUNLOCK is one such flag. However it is exclusively used by
> try_to_munlock() which specifies no other flags. Therefore rather than
> overload try_to_unmap_one() with unrelated behaviour split this out into
> it's own function and remove the flag.
> 
> Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
> Reviewed-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> 
> ---
> 
> v9:
> * Improved comments
> 
> v8:
> * Renamed try_to_munlock to page_mlock to better reflect what the
>   function actually does.
> * Removed the TODO from the documentation that this patch addresses.
> 
> v7:
> * Added Christoph's Reviewed-by
> 
> v4:
> * Removed redundant check for VM_LOCKED
> ---
>  Documentation/vm/unevictable-lru.rst | 33 ++++++---------
>  include/linux/rmap.h                 |  3 +-
>  mm/mlock.c                           | 10 ++---
>  mm/rmap.c                            | 61 ++++++++++++++++++++--------
>  4 files changed, 63 insertions(+), 44 deletions(-)
> 
> diff --git a/Documentation/vm/unevictable-lru.rst b/Documentation/vm/unevictable-lru.rst
> index 0e1490524f53..eae3af17f2d9 100644
> --- a/Documentation/vm/unevictable-lru.rst
> +++ b/Documentation/vm/unevictable-lru.rst
> @@ -389,14 +389,14 @@ mlocked, munlock_vma_page() updates that zone statistics for the number of
>  mlocked pages.  Note, however, that at this point we haven't checked whether
>  the page is mapped by other VM_LOCKED VMAs.
>  
> -We can't call try_to_munlock(), the function that walks the reverse map to
> +We can't call page_mlock(), the function that walks the reverse map to
>  check for other VM_LOCKED VMAs, without first isolating the page from the LRU.
> -try_to_munlock() is a variant of try_to_unmap() and thus requires that the page
> +page_mlock() is a variant of try_to_unmap() and thus requires that the page
>  not be on an LRU list [more on these below].  However, the call to
> -isolate_lru_page() could fail, in which case we couldn't try_to_munlock().  So,
> +isolate_lru_page() could fail, in which case we can't call page_mlock().  So,
>  we go ahead and clear PG_mlocked up front, as this might be the only chance we
> -have.  If we can successfully isolate the page, we go ahead and
> -try_to_munlock(), which will restore the PG_mlocked flag and update the zone
> +have.  If we can successfully isolate the page, we go ahead and call
> +page_mlock(), which will restore the PG_mlocked flag and update the zone
>  page statistics if it finds another VMA holding the page mlocked.  If we fail
>  to isolate the page, we'll have left a potentially mlocked page on the LRU.
>  This is fine, because we'll catch it later if and if vmscan tries to reclaim
> @@ -545,31 +545,24 @@ munlock or munmap system calls, mm teardown (munlock_vma_pages_all), reclaim,
>  holepunching, and truncation of file pages and their anonymous COWed pages.
>  
>  
> -try_to_munlock() Reverse Map Scan
> +page_mlock() Reverse Map Scan
>  ---------------------------------
>  
> -.. warning::
> -   [!] TODO/FIXME: a better name might be page_mlocked() - analogous to the
> -   page_referenced() reverse map walker.
> -
>  When munlock_vma_page() [see section :ref:`munlock()/munlockall() System Call
>  Handling <munlock_munlockall_handling>` above] tries to munlock a
>  page, it needs to determine whether or not the page is mapped by any
>  VM_LOCKED VMA without actually attempting to unmap all PTEs from the
>  page.  For this purpose, the unevictable/mlock infrastructure
> -introduced a variant of try_to_unmap() called try_to_munlock().
> +introduced a variant of try_to_unmap() called page_mlock().
>  
> -try_to_munlock() calls the same functions as try_to_unmap() for anonymous and
> -mapped file and KSM pages with a flag argument specifying unlock versus unmap
> -processing.  Again, these functions walk the respective reverse maps looking
> -for VM_LOCKED VMAs.  When such a VMA is found, as in the try_to_unmap() case,
> -the functions mlock the page via mlock_vma_page() and return SWAP_MLOCK.  This
> -undoes the pre-clearing of the page's PG_mlocked done by munlock_vma_page.
> +page_mlock() walks the respective reverse maps looking for VM_LOCKED VMAs. When
> +such a VMA is found the page is mlocked via mlock_vma_page(). This undoes the
> +pre-clearing of the page's PG_mlocked done by munlock_vma_page.
>  
> -Note that try_to_munlock()'s reverse map walk must visit every VMA in a page's
> +Note that page_mlock()'s reverse map walk must visit every VMA in a page's
>  reverse map to determine that a page is NOT mapped into any VM_LOCKED VMA.
>  However, the scan can terminate when it encounters a VM_LOCKED VMA.
> -Although try_to_munlock() might be called a great many times when munlocking a
> +Although page_mlock() might be called a great many times when munlocking a
>  large region or tearing down a large address space that has been mlocked via
>  mlockall(), overall this is a fairly rare event.
>  
> @@ -602,7 +595,7 @@ inactive lists to the appropriate node's unevictable list.
>  shrink_inactive_list() should only see SHM_LOCK'd pages that became SHM_LOCK'd
>  after shrink_active_list() had moved them to the inactive list, or pages mapped
>  into VM_LOCKED VMAs that munlock_vma_page() couldn't isolate from the LRU to
> -recheck via try_to_munlock().  shrink_inactive_list() won't notice the latter,
> +recheck via page_mlock().  shrink_inactive_list() won't notice the latter,
>  but will pass on to shrink_page_list().
>  
>  shrink_page_list() again culls obviously unevictable pages that it could
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index def5c62c93b3..38a746787c2f 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -87,7 +87,6 @@ struct anon_vma_chain {
>  
>  enum ttu_flags {
>  	TTU_MIGRATION		= 0x1,	/* migration mode */
> -	TTU_MUNLOCK		= 0x2,	/* munlock mode */
>  
>  	TTU_SPLIT_HUGE_PMD	= 0x4,	/* split huge PMD if any */
>  	TTU_IGNORE_MLOCK	= 0x8,	/* ignore mlock */
> @@ -239,7 +238,7 @@ int page_mkclean(struct page *);
>   * called in munlock()/munmap() path to check for other vmas holding
>   * the page mlocked.
>   */
> -void try_to_munlock(struct page *);
> +void page_mlock(struct page *page);
>  
>  void remove_migration_ptes(struct page *old, struct page *new, bool locked);
>  
> diff --git a/mm/mlock.c b/mm/mlock.c
> index df590fda5688..a518d4c48e65 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -108,7 +108,7 @@ void mlock_vma_page(struct page *page)
>  /*
>   * Finish munlock after successful page isolation
>   *
> - * Page must be locked. This is a wrapper for try_to_munlock()
> + * Page must be locked. This is a wrapper for page_mlock()
>   * and putback_lru_page() with munlock accounting.
>   */
>  static void __munlock_isolated_page(struct page *page)
> @@ -118,7 +118,7 @@ static void __munlock_isolated_page(struct page *page)
>  	 * and we don't need to check all the other vmas.
>  	 */
>  	if (page_mapcount(page) > 1)
> -		try_to_munlock(page);
> +		page_mlock(page);
>  
>  	/* Did try_to_unlock() succeed or punt? */
>  	if (!PageMlocked(page))
> @@ -158,7 +158,7 @@ static void __munlock_isolation_failed(struct page *page)
>   * munlock()ed or munmap()ed, we want to check whether other vmas hold the
>   * page locked so that we can leave it on the unevictable lru list and not
>   * bother vmscan with it.  However, to walk the page's rmap list in
> - * try_to_munlock() we must isolate the page from the LRU.  If some other
> + * page_mlock() we must isolate the page from the LRU.  If some other
>   * task has removed the page from the LRU, we won't be able to do that.
>   * So we clear the PageMlocked as we might not get another chance.  If we
>   * can't isolate the page, we leave it for putback_lru_page() and vmscan
> @@ -168,7 +168,7 @@ unsigned int munlock_vma_page(struct page *page)
>  {
>  	int nr_pages;
>  
> -	/* For try_to_munlock() and to serialize with page migration */
> +	/* For page_mlock() and to serialize with page migration */
>  	BUG_ON(!PageLocked(page));
>  	VM_BUG_ON_PAGE(PageTail(page), page);
>  
> @@ -205,7 +205,7 @@ static int __mlock_posix_error_return(long retval)
>   *
>   * The fast path is available only for evictable pages with single mapping.
>   * Then we can bypass the per-cpu pvec and get better performance.
> - * when mapcount > 1 we need try_to_munlock() which can fail.
> + * when mapcount > 1 we need page_mlock() which can fail.
>   * when !page_evictable(), we need the full redo logic of putback_lru_page to
>   * avoid leaving evictable page in unevictable list.
>   *
> diff --git a/mm/rmap.c b/mm/rmap.c
> index bc08c4d4b58a..e88966903e1e 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1405,10 +1405,6 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>  	struct mmu_notifier_range range;
>  	enum ttu_flags flags = (enum ttu_flags)(long)arg;
>  
> -	/* munlock has nothing to gain from examining un-locked vmas */
> -	if ((flags & TTU_MUNLOCK) && !(vma->vm_flags & VM_LOCKED))
> -		return true;
> -
>  	if (IS_ENABLED(CONFIG_MIGRATION) && (flags & TTU_MIGRATION) &&
>  	    is_zone_device_page(page) && !is_device_private_page(page))
>  		return true;
> @@ -1469,8 +1465,6 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>  				page_vma_mapped_walk_done(&pvmw);
>  				break;
>  			}
> -			if (flags & TTU_MUNLOCK)
> -				continue;
>  		}
>  
>  		/* Unexpected PMD-mapped THP? */
> @@ -1784,20 +1778,53 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
>  	return !page_mapcount(page) ? true : false;
>  }
>  
> +/*
> + * Walks the vma's mapping a page and mlocks the page if any locked vma's are
> + * found. Once one is found the page is locked and the scan can be terminated.
> + */

Can you please add that this requires the mmap_sem() lock to the
comments?

> +static bool page_mlock_one(struct page *page, struct vm_area_struct *vma,
> +				 unsigned long address, void *unused)
> +{
> +	struct page_vma_mapped_walk pvmw = {
> +		.page = page,
> +		.vma = vma,
> +		.address = address,
> +	};
> +
> +	/* An un-locked vma doesn't have any pages to lock, continue the scan */
> +	if (!(vma->vm_flags & VM_LOCKED))
> +		return true;
> +
> +	while (page_vma_mapped_walk(&pvmw)) {
> +		/* PTE-mapped THP are never mlocked */
> +		if (!PageTransCompound(page))
> +			mlock_vma_page(page);
> +		page_vma_mapped_walk_done(&pvmw);
> +
> +		/*
> +		 * no need to continue scanning other vma's if the page has
> +		 * been locked.
> +		 */
> +		return false;
> +	}
> +
> +	return true;
> +}
> +
>  /**
> - * try_to_munlock - try to munlock a page
> - * @page: the page to be munlocked
> + * page_mlock - try to mlock a page
> + * @page: the page to be mlocked
>   *
> - * Called from munlock code.  Checks all of the VMAs mapping the page
> - * to make sure nobody else has this page mlocked. The page will be
> - * returned with PG_mlocked cleared if no other vmas have it mlocked.
> + * Called from munlock code. Checks all of the VMAs mapping the page and mlocks
> + * the page if any are found. The page will be returned with PG_mlocked cleared
> + * if it is not mapped by any locked vmas.
> + *
> + * mmap_lock should be held for read or write.
>   */
> -
> -void try_to_munlock(struct page *page)
> +void page_mlock(struct page *page)
>  {
>  	struct rmap_walk_control rwc = {
> -		.rmap_one = try_to_unmap_one,
> -		.arg = (void *)TTU_MUNLOCK,
> +		.rmap_one = page_mlock_one,
>  		.done = page_not_mapped,
>  		.anon_lock = page_lock_anon_vma_read,
>  
> @@ -1849,7 +1876,7 @@ static struct anon_vma *rmap_walk_anon_lock(struct page *page,
>   * Find all the mappings of a page using the mapping pointer and the vma chains
>   * contained in the anon_vma struct it points to.
>   *
> - * When called from try_to_munlock(), the mmap_lock of the mm containing the vma
> + * When called from page_mlock(), the mmap_lock of the mm containing the vma
>   * where the page was found will be held for write.  So, we won't recheck
>   * vm_flags for that VMA.  That should be OK, because that vma shouldn't be
>   * LOCKED.
> @@ -1901,7 +1928,7 @@ static void rmap_walk_anon(struct page *page, struct rmap_walk_control *rwc,
>   * Find all the mappings of a page using the mapping pointer and the vma chains
>   * contained in the address_space struct it points to.
>   *
> - * When called from try_to_munlock(), the mmap_lock of the mm containing the vma
> + * When called from page_mlock(), the mmap_lock of the mm containing the vma
>   * where the page was found will be held for write.  So, we won't recheck
>   * vm_flags for that VMA.  That should be OK, because that vma shouldn't be
>   * LOCKED.
> -- 
> 2.20.1
> 
> 

I believe munlock_vma_pages_range() still references the old function
name?

Thanks,
Liam