Re: [PATCH RFC v2 1/2] mm: Don't skip swap entry even if zap_details specified

[Alistair Popple]

On Monday, 10 January 2022 7:37:15 PM AEDT David Hildenbrand wrote:
> On 15.11.21 14:49, Peter Xu wrote:
> > This check existed since the 1st git commit of Linux repository, but at that
> > time there's no page migration yet so I think it's okay.
> > 
> > With page migration enabled, it should logically be possible that we zap some
> > shmem pages during migration.  When that happens, IIUC the old code could have
> > the RSS counter accounted wrong on MM_SHMEMPAGES because we will zap the ptes
> > without decreasing the counters for the migrating entries.  I have no unit test
> > to prove it as I don't know an easy way to trigger this condition, though.
> > 
> > Besides, the optimization itself is already confusing IMHO to me in a few points:
> > 
> >   - The wording "skip swap entries" is confusing, because we're not skipping all
> >     swap entries - we handle device private/exclusive pages before that.
> 
> I think one part of the confusion is "swap vs non-swap" entries.
> For !pte_none() && !pte_present() we can have
> 
> * swap entry
> * non-swap entry
> ** device exclusive entry
> ** device private entry
> ** HWpoison entry
> ** migration entry
> 
> So the comment claims to skip "swap entries" but also skips HWpoison and
> migration entries, and I think that's the confusing part.
> Both only apply to PageAnon().

I must be missing something but why do these only apply to PageAnon()?

> IIUC, the only way we could get details != NULL is via unmap_mapping_page()+unmap_mapping_pages().
> 
> I do wonder if any of the callers really cares about PageAnon() pages where this would be relevant.
> 
> Am I wrong or is unmap_mapping_pages() never called with "even_cows == true" and we can remove
> that paremeter:

Except that unmap_mapping_range() takes `even_cows` as a parameter and passes
that to unmap_mapping_pages(), and from what I can tell there are callers of
unmap_mapping_range() that set `even_cows = true`.

> git grep -C2 unmap_mapping_pages
> fs/afs/callback.c-      struct afs_vnode *vnode = container_of(work, struct afs_vnode, cb_work);
> fs/afs/callback.c-
> fs/afs/callback.c:      unmap_mapping_pages(vnode->vfs_inode.i_mapping, 0, 0, false);
> fs/afs/callback.c-}
> fs/afs/callback.c-
> --
> fs/dax.c-               if (dax_is_zero_entry(entry)) {
> fs/dax.c-                       xas_unlock_irq(xas);
> fs/dax.c:                       unmap_mapping_pages(mapping,
> fs/dax.c-                                       xas->xa_index & ~PG_PMD_COLOUR,
> fs/dax.c-                                       PG_PMD_NR, false);
> --
> fs/dax.c-        * get_user_pages() slow path.  The slow path is protected by
> fs/dax.c-        * pte_lock() and pmd_lock(). New references are not taken without
> fs/dax.c:        * holding those locks, and unmap_mapping_pages() will not zero the
> fs/dax.c-        * pte or pmd without holding the respective lock, so we are
> fs/dax.c-        * guaranteed to either see new references or prevent new
> fs/dax.c-        * references from being established.
> fs/dax.c-        */
> fs/dax.c:       unmap_mapping_pages(mapping, start_idx, end_idx - start_idx + 1, 0);
> fs/dax.c-
> fs/dax.c-       xas_lock_irq(&xas);
> --
> fs/dax.c-               /* we are replacing a zero page with block mapping */
> fs/dax.c-               if (dax_is_pmd_entry(entry))
> fs/dax.c:                       unmap_mapping_pages(mapping, index & ~PG_PMD_COLOUR,
> fs/dax.c-                                       PG_PMD_NR, false);
> fs/dax.c-               else /* pte entry */
> fs/dax.c:                       unmap_mapping_pages(mapping, index, 1, false);
> fs/dax.c-       }
> fs/dax.c-
> --
> include/linux/mm.h-                         bool *unlocked);
> include/linux/mm.h-void unmap_mapping_page(struct page *page);
> include/linux/mm.h:void unmap_mapping_pages(struct address_space *mapping,
> include/linux/mm.h-             pgoff_t start, pgoff_t nr, bool even_cows);
> include/linux/mm.h-void unmap_mapping_range(struct address_space *mapping,
> --
> include/linux/mm.h-}
> include/linux/mm.h-static inline void unmap_mapping_page(struct page *page) { }
> include/linux/mm.h:static inline void unmap_mapping_pages(struct address_space *mapping,
> include/linux/mm.h-             pgoff_t start, pgoff_t nr, bool even_cows) { }
> include/linux/mm.h-static inline void unmap_mapping_range(struct address_space *mapping,
> --
> mm/khugepaged.c-
> mm/khugepaged.c-                if (page_mapped(page))
> mm/khugepaged.c:                        unmap_mapping_pages(mapping, index, 1, false);
> mm/khugepaged.c-
> mm/khugepaged.c-                xas_lock_irq(&xas);
> --
> mm/memory.c- * Unmap this page from any userspace process which still has it mmaped.
> mm/memory.c- * Typically, for efficiency, the range of nearby pages has already been
> mm/memory.c: * unmapped by unmap_mapping_pages() or unmap_mapping_range().  But once
> mm/memory.c- * truncation or invalidation holds the lock on a page, it may find that
> mm/memory.c- * the page has been remapped again: and then uses unmap_mapping_page()
> --
> mm/memory.c-
> mm/memory.c-/**
> mm/memory.c: * unmap_mapping_pages() - Unmap pages from processes.
> mm/memory.c- * @mapping: The address space containing pages to be unmapped.
> mm/memory.c- * @start: Index of first page to be unmapped.
> --
> mm/memory.c- * cache.
> mm/memory.c- */
> mm/memory.c:void unmap_mapping_pages(struct address_space *mapping, pgoff_t start,
> mm/memory.c-            pgoff_t nr, bool even_cows)
> mm/memory.c-{
> --
> mm/memory.c-    i_mmap_unlock_write(mapping);
> mm/memory.c-}
> mm/memory.c:EXPORT_SYMBOL_GPL(unmap_mapping_pages);
> mm/memory.c-
> mm/memory.c-/**
> --
> mm/memory.c-    }
> mm/memory.c-
> mm/memory.c:    unmap_mapping_pages(mapping, hba, hlen, even_cows);
> mm/memory.c-}
> mm/memory.c-EXPORT_SYMBOL(unmap_mapping_range);
> --
> mm/truncate.c-                           * zap the rest of the file in one hit.
> mm/truncate.c-                           */
> mm/truncate.c:                          unmap_mapping_pages(mapping, index,
> mm/truncate.c-                                          (1 + end - index), false);
> mm/truncate.c-                          did_range_unmap = 1;
> --
> mm/truncate.c-   */
> mm/truncate.c-  if (dax_mapping(mapping)) {
> mm/truncate.c:          unmap_mapping_pages(mapping, start, end - start + 1, false);
> mm/truncate.c-  }
> mm/truncate.c-out:
> 
> 
>