Re: [PATCH] btrfs: limit the number of asynchronous delalloc pages to reasonable value

[Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>]

hello,

On 11/22/2016 12:39 AM, David Sterba wrote:
> On Tue, Nov 08, 2016 at 05:30:58PM +0800, Wang Xiaoguang wrote:
>> The current limit of number of asynchronous delalloc pages is (10 * SZ_1M).
>> For 4K page, the total ram bytes would be 40G, very big value, I think in
>> most cases, this limit will not work, here I set limit of the number of
>> asynchronous delalloc pages to SZ_1M(4GB ram bytes).
>>
>> Signed-off-by: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
>> ---
>>   fs/btrfs/inode.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
>> index 8e3a5a2..3a910f6 100644
>> --- a/fs/btrfs/inode.c
>> +++ b/fs/btrfs/inode.c
>> @@ -1158,7 +1158,7 @@ static int cow_file_range_async(struct inode *inode, struct page *locked_page,
>>   	struct btrfs_root *root = BTRFS_I(inode)->root;
>>   	unsigned long nr_pages;
>>   	u64 cur_end;
>> -	int limit = 10 * SZ_1M;
>> +	int limit = SZ_1M;
> This looks like mismatch in units, the limit seems to be "10MiB" but
> it's used to compare async_delalloc_pages, which is in pages. I'm not
> sure what's the expected value for limit, as 10MiB is too small.
There are such codes in cow_file_range_async()
     nr_pages = (cur_end - start + PAGE_SIZE) >> PAGE_SHIFT;
     atomic_add(nr_pages, &root->fs_info->async_delalloc_pages);

So here the real limit is 10 * SZ_1M * 4096, 40GB memory, I think this
value is too big, so modify it to 4GB.

Regards,
Xiaoguang Wang
>
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