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SubjectRe: [PATCH v2 05/12] mm: Add and use find_lock_entries
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...

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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