On 2018年04月24日 13:59, Nikolay Borisov wrote:
>
>
> On 24.04.2018 02:03, David Sterba wrote:
>> The eb length is nodesize, as initialized in __alloc_extent_buffer.
>> Regardless of start, we should always get the same number of pages, so
>> use that fact.
>>
>> Signed-off-by: David Sterba <dste...@suse.com>
>> ---
>> fs/btrfs/extent_io.h | 3 +--
>> 1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
>> index a53009694b16..ee92c1289edd 100644
>> --- a/fs/btrfs/extent_io.h
>> +++ b/fs/btrfs/extent_io.h
>> @@ -454,8 +454,7 @@ void wait_on_extent_buffer_writeback(struct
>> extent_buffer *eb);
>>
>> static inline unsigned long num_extent_pages(u64 start, u64 len)
>> {
>> - return ((start + len + PAGE_SIZE - 1) >> PAGE_SHIFT) -
>> - (start >> PAGE_SHIFT);
>> + return len >> PAGE_SHIFT;
>
> Shouldn't this really be len + PAGE_SIZE -1 or in fact DIV_ROUND_DOWN
> (len, PAGE_SIZE). Because with a nodesize of 4k (and basically less than
> a page size) we can get into a situation where we do:
>
> 4096 >> 13 = 0
>
> On powerpc for example we have:
>
> arch/powerpc/include/asm/page.h:#define PAGE_SHIFT 18
> arch/powerpc/include/asm/page.h:#define PAGE_SHIFT 16
> arch/powerpc/include/asm/page.h:#define PAGE_SHIFT 14
For such case, the fs won't be mounted as we don't have sub-pagesized
nodesize support yet.
So won't hit the problem.
Although a WARN_ON(len < PAGE_SIZE || IS_ALIGNED(start, PAGE_SIZE))
would do no harm here.
Thanks,
Qu
> arch/powerpc/include/asm/page.h:#define PAGE_SHIFT 12
>
>
>> }
>>
>> static inline void extent_buffer_get(struct extent_buffer *eb)
>>
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