Full buffer cache not working properly

Full buffer cache not working properly

[Thanos Makatos]

I run several times the following test and what I've seen is that when 
the buffer cache becomes full, unneeded dirty buffer heads are not evicted
and no other memory allocation can happen (including reading a block 
from the disk to the buffer cache). Should this happen?
Here's the code that reproduces this issue:

for(i = 0; i < get_capacity(bdev->bd_disk)/8; i++) {
        bh = __bread(bdev, i, PAGE_SIZE);
        lock_buffer(bh);
        memset(bh->b_data, 0, PAGE_SIZE);
        set_buffer_dirty(bh);
        unlock_buffer(bh);
        __brelse(bh);
}

I added some printks printing the buffer head's reference counter: after 
the '__bread' ref count is 2, after '__brelse' ref count is 1. Shouldn't 
these
numbers be 1 and 0 respectively? I added an extra 'put_bh' before (or 
after) the '__brelse' but I get a BUG().

Re: Full buffer cache not working properly

[Andrew Morton]

On Thu, 12 Jul 2007 17:03:34 +0300 Thanos Makatos <mcatos@ics.forth.gr> wrote:

> I run several times the following test and what I've seen is that when 
> the buffer cache becomes full, unneeded dirty buffer heads are not evicted
> and no other memory allocation can happen (including reading a block 
> from the disk to the buffer cache). Should this happen?
> Here's the code that reproduces this issue:
> 
> for(i = 0; i < get_capacity(bdev->bd_disk)/8; i++) {
>         bh = __bread(bdev, i, PAGE_SIZE);
>         lock_buffer(bh);
>         memset(bh->b_data, 0, PAGE_SIZE);
>         set_buffer_dirty(bh);
>         unlock_buffer(bh);
>         __brelse(bh);
> }
> 
> I added some printks printing the buffer head's reference counter: after 
> the '__bread' ref count is 2, after '__brelse' ref count is 1. Shouldn't 
> these
> numbers be 1 and 0 respectively? I added an extra 'put_bh' before (or 
> after) the '__brelse' but I get a BUG().

You've been very unkind to the VFS.  Put a balance_dirty_pages() call in
that loop to give the system an opportunity to do some writeback and
throttling.