Re: [PATCHv2] mm/compaction: Break out of loop on !PageBuddy in isolate_freepages_block

[Bartlomiej Zolnierkiewicz]

Hi,

On Thursday, March 06, 2014 10:21:32 AM Laura Abbott wrote:
> We received several reports of bad page state when freeing CMA pages
> previously allocated with alloc_contig_range:
> 
> <1>[ 1258.084111] BUG: Bad page state in process Binder_A  pfn:63202
> <1>[ 1258.089763] page:d21130b0 count:0 mapcount:1 mapping:  (null) index:0x7dfbf
> <1>[ 1258.096109] page flags: 0x40080068(uptodate|lru|active|swapbacked)
> 
> Based on the page state, it looks like the page was still in use. The page
> flags do not make sense for the use case though. Further debugging showed
> that despite alloc_contig_range returning success, at least one page in the
> range still remained in the buddy allocator.
> 
> There is an issue with isolate_freepages_block. In strict mode (which CMA
> uses), if any pages in the range cannot be isolated,
> isolate_freepages_block should return failure 0. The current check keeps
> track of the total number of isolated pages and compares against the size
> of the range:
> 
>         if (strict && nr_strict_required > total_isolated)
>                 total_isolated = 0;
> 
> After taking the zone lock, if one of the pages in the range is not
> in the buddy allocator, we continue through the loop and do not
> increment total_isolated. If in the last iteration of the loop we isolate
> more than one page (e.g. last page needed is a higher order page), the
> check for total_isolated may pass and we fail to detect that a page was
> skipped. The fix is to bail out if the loop immediately if we are in
> strict mode. There's no benfit to continuing anyway since we need all
> pages to be isolated. Additionally, drop the error checking based on
> nr_strict_required and just check the pfn ranges. This matches with
> what isolate_freepages_range does.
> 
> Signed-off-by: Laura Abbott <lauraa@xxxxxxxxxxxxxx>

Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx>

Thanks for catching & fixing this!

Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung R&D Institute Poland
Samsung Electronics

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