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SubjectRe: [PATCH] mm: fix long time stall from mm_populate
On Mon 10-02-20 16:19:58, Minchan Kim wrote:
> Basically, fault handler releases mmap_sem before requesting readahead
> and then it is supposed to retry lookup the page from page cache with
> FAULT_FLAG_TRIED so that it avoids the live lock of infinite retry.
>
> However, what happens if the fault handler find a page from page
> cache and the page has readahead marker but are waiting under
> writeback? Plus one more condition, it happens under mm_populate
> which repeats faulting unless it encounters error. So let's assemble
> conditions below.
>
> __mm_populate
> for (...)
> get_user_pages(faluty_address)
> handle_mm_fault
> filemap_fault
> find a page form page(PG_uptodate|PG_readahead|PG_writeback)
> it will return VM_FAULT_RETRY
> continue with faulty_address
>
> IOW, it will repeat fault retry logic until the page will be written
> back in the long run. It makes big spike latency of several seconds.
>
> This patch solves the issue by turning off fault retry logic in second
> trial.
>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
> It was orignated from code review once I have seen several user reports
> but didn't confirm yet it's the root cause.

Yes, I think the immediate problem is actually elsewhere but I agree that
__mm_populate() should follow the general protocol of retrying only once
so your change should make it more robust. The patch looks good to me, you
can add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

>
> mm/gup.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 1b521e0ac1de..b3f825092abf 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1196,6 +1196,7 @@ int __mm_populate(unsigned long start, unsigned long len, int ignore_errors)
> struct vm_area_struct *vma = NULL;
> int locked = 0;
> long ret = 0;
> + bool tried = false;
>
> end = start + len;
>
> @@ -1226,14 +1227,18 @@ int __mm_populate(unsigned long start, unsigned long len, int ignore_errors)
> * double checks the vma flags, so that it won't mlock pages
> * if the vma was already munlocked.
> */
> - ret = populate_vma_page_range(vma, nstart, nend, &locked);
> + ret = populate_vma_page_range(vma, nstart, nend,
> + tried ? NULL : &locked);
> if (ret < 0) {
> if (ignore_errors) {
> ret = 0;
> continue; /* continue at next VMA */
> }
> break;
> - }
> + } else if (ret == 0)
> + tried = true;
> + else
> + tried = false;
> nend = nstart + ret * PAGE_SIZE;
> ret = 0;
> }
> --
> 2.25.0.225.g125e21ebc7-goog
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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