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

[David Sterba <dsterba@suse.cz>]

On Tue, Nov 22, 2016 at 05:20:10PM +0800, Wang Xiaoguang wrote:
> 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.

Yeah, but I don't see why 4GB would be a good value either.