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SubjectRe: [PATCH] mm: ratelimit end_swap_bio_write() error
On (01/08/18 19:22), Sergey Senozhatsky wrote:
[..]
> > Your changelog is rather modest on the information.
>
> fair point!
>
> > Could you be more specific on how the problem actually happens how
> > likely it is?
>
> ok. so what we have is
>
> slow_path / swap-out page
> __zram_bvec_write(page)
> compressed_page = zcomp_compress(page)
> zs_malloc(compressed_page)
> // no available zspage found, need to allocate new
> alloc_zspage()
> {
> for (i = 0; i < class->pages_per_zspage; i++)
> page = alloc_page(gfp);
> if (!page)
> return NULL
> }
>
> return -ENOMEM
> ...
> printk("Write-error on swap-device...");
>
>
> zspage-s can consist of up to ->pages_per_zspage normal pages.
> if alloc_page() fails then we can't allocate the entire zspage,
> so we can't store the swapped out page, so it remains in ram
> and we don't make any progress. so we try to swap another page
> and may be do the whole zs_malloc()->alloc_zspage() again, may
> be not. depending on how bad the OOM situation is there can be
> few or many "Write-error on swap-device" errors.
>
> > And again, I do not think the throttling is an appropriate counter
> > measure. We do want to print those messages when a critical situation
> > happens. If we have a fallback then simply do not print at all.
>
> sure, but with the ratelimited printk we still print those messages.
> we just don't print it for every single page we failed to write
> to the device. the existing error messages can (*sometimes*) be noisy
> and not very informative - "Write-error on swap-device (%u:%u:%llu)\n";
> it's not like 1000 of those tell more than 1 or 10.

Michal, does that make sense? with the updated/reworked commit
message will the patch be good enough?

-ss

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