Re: [RFC PATCH 2/2] fs/dcache: Add negative-dentry-ratio config

[Stephen Brennan <stephen.s.brennan@oracle.com>]

On 3/31/22 12:45, Al Viro wrote:
> On Thu, Mar 31, 2022 at 12:08:27PM -0700, Stephen Brennan wrote:
>> Negative dentry bloat is a well-known problem. For systems without
>> memory pressure, some workloads (like repeated stat calls) can create an
>> unbounded amount of negative dentries quite quickly. In the best case,
>> these dentries could speed up a subsequent name lookup, but in the worst
>> case, they are never used and their memory never freed.
>>
>> While systems without memory pressure may not need that memory for other
>> purposes, negative dentry bloat can have other side-effects, such as
>> soft lockups when traversing the d_subdirs list or general slowness with
>> managing them. It is a good idea to have some sort of mechanism for
>> controlling negative dentries, even outside memory pressure.
>>
>> This patch attempts to do so in a fair way. Workloads which create many
>> negative dentries must create many dentries, or convert dentries from
>> positive to negative. Thus, negative dentry management is best done
>> during these same operations, as it will amortize its cost, and
>> distribute the cost to the perpetrators of the dentry bloat. We
>> introduce a sysctl "negative-dentry-ratio" which sets a maximum number
>> of negative dentries per positive dentry, N:1. When a dentry is created
>> or unlinked, the next N+1 dentries of the parent are scanned. If no
>> positive dentries are found, then a candidate negative dentry is killed.
> 
> Er...  So what's to stop d_move() from leaving you with your cursor
> pointer poiting into the list of children of another parent?
> 
> What's more, your dentry_unlist() logics will be defeated by that -
> if victim used to have a different parent, got moved, then evicted,
> it looks like you could end up with old parent cursor pointing
> to the victim and left unmodified by dentry_unlist() (since it looks
> only at the current parent's cursor).  Wait for it to be freed and
> voila - access to old parent's cursor will do unpleasant things.
> 
> What am I missing here?

Thanks for this catch. Since d_move holds the parent's lock, it should
be possible to include the same condition as dentry_unlist() to ensure
the cursor gets advanced if necessary. I could make it a small inline
helper to make things easier to read. I will go ahead and fix that.

Thanks,
Stephen