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SubjectRe: Full buffer cache not working properly
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.

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