linux-kernel - Re: [RFC PATCH] cifs: Fix credit handling in cifs_io

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Message-ID: <581217.1716475935@warthog.procyon.org.uk>
Date: Thu, 23 May 2024 15:52:15 +0100
From: David Howells <dhowells@...hat.com>
To: Tom Talpey <tom@...pey.com>
Cc: dhowells@...hat.com, Steve French <sfrench@...ba.org>,
    Paulo Alcantara <pc@...guebit.com>,
    Shyam Prasad N <nspmangalore@...il.com>,
    Rohith Surabattula <rohiths.msft@...il.com>,
    Jeff Layton <jlayton@...nel.org>, linux-cifs@...r.kernel.org,
    netfs@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
    linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] cifs: Fix credit handling in cifs_io_subrequest cleanup

Tom Talpey <tom@...pey.com> wrote:

> From a protocol standpoint it's correct to reserve the credits while the
> operation is in flight. But from a code standpoint it seems risky to
> stop accounting for them. What if the operation is canceled, or times
> out?

No idea, TBH - SMB credits wrangling isn't my area of expertise.  Note the
patch is superfluous as smb2_readv/writev_callback() clear the credits at the
end; worse, it's actually wrong as we're not allowed to touch [rw]data after
calling ->async_readv()/->async_writev().

> I'd quibble with the assertion that the server will "give us new credits
> in the response". The number of granted credits is always the server's
> decision, not guaranteed by the protocol (except for certain edge
> conditions).

It does give us new credits in the response, doesn't it?  In
hdr.CreditRequest - though I suppose this could be zero.

> I guess I'd suggest a deeper review by someone familiar with the
> mechanics of fs/smb/client credit accounting. It might be ok!

I've given Steve a patch to try and find where the double add occurs.

David