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SubjectRe: [PATCH v2 05/12] mm: Add and use find_lock_entries
On Tue, Sep 29, 2020 at 10:58:55AM +0200, Jan Kara wrote:
> On Mon 14-09-20 14:00:35, Matthew Wilcox (Oracle) wrote:
> > We have three functions (shmem_undo_range(), truncate_inode_pages_range()
> > and invalidate_mapping_pages()) which want exactly this function, so
> > add it to filemap.c.
> >
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > diff --git a/mm/shmem.c b/mm/shmem.c
> ...
> > index b65263d9bb67..a73ce8ce28e3 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -905,12 +905,8 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
> >
> > pagevec_init(&pvec);
> > index = start;
> > - while (index < end) {
> > - pvec.nr = find_get_entries(mapping, index,
> > - min(end - index, (pgoff_t)PAGEVEC_SIZE),
> > - pvec.pages, indices);
> > - if (!pvec.nr)
> > - break;
> > + while (index < end && find_lock_entries(mapping, index, end - 1,
> > + &pvec, indices)) {
> > for (i = 0; i < pagevec_count(&pvec); i++) {
> > struct page *page = pvec.pages[i];
> >
> > @@ -925,18 +921,10 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
> > index, page);
> > continue;
> > }
> > + index += thp_nr_pages(page) - 1;
> >
> > - VM_BUG_ON_PAGE(page_to_pgoff(page) != index, page);
> > -
> > - if (!trylock_page(page))
> > - continue;
> > -
> > - if ((!unfalloc || !PageUptodate(page)) &&
> > - page_mapping(page) == mapping) {
> > - VM_BUG_ON_PAGE(PageWriteback(page), page);
> > - if (shmem_punch_compound(page, start, end))
> > - truncate_inode_page(mapping, page);
> > - }
> > + if (!unfalloc || !PageUptodate(page))
> > + truncate_inode_page(mapping, page);
>
> Is dropping shmem_punch_compound() really safe? AFAICS it can also call
> split_huge_page() which will try to split THP to be able to truncate it.
> That being said there's another loop in shmem_undo_range() which will try
> again so what you did might make a difference with performance but not much
> else. But still it would be good to at least comment about this in the
> changelog...

OK, I need to provide better argumentation in the changelog.

shmem_punch_compound() handles partial THPs. By the end of this series,
we handle the partial pages in the next part of the function ... the
part where we're handling partial PAGE_SIZE pages. At this point in
the series, it's safe to remove the shmem_punch_compound() call because
the new find_lock_entries() loop will only return THPs that lie entirely
within the range.

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