netdev - Re: [PATCH net-next] tls: cap the output scatter list to something reasonable

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Date:   Mon, 7 Feb 2022 09:26:19 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, borisp@...dia.com,
        john.fastabend@...il.com, daniel@...earbox.net,
        vfedorenko@...ek.ru, kernel-team@...com, axboe@...nel.dk
Subject: Re: [PATCH net-next] tls: cap the output scatter list to something
 reasonable

On Mon, 7 Feb 2022 17:15:30 +0000 Al Viro wrote:
> On Wed, Feb 02, 2022 at 02:20:31PM -0800, Jakub Kicinski wrote:
> > TLS recvmsg() passes user pages as destination for decrypt.
> > The decrypt operation is repeated record by record, each
> > record being 16kB, max. TLS allocates an sg_table and uses
> > iov_iter_get_pages() to populate it with enough pages to
> > fit the decrypted record.
> > 
> > Even though we decrypt a single message at a time we size
> > the sg_table based on the entire length of the iovec.
> > This leads to unnecessarily large allocations, risking
> > triggering OOM conditions.
> > 
> > Use iov_iter_truncate() / iov_iter_reexpand() to construct
> > a "capped" version of iov_iter_npages(). Alternatively we
> > could parametrize iov_iter_npages() to take the size as
> > arg instead of using i->count, or do something else..  
> 
> Er...  Would simply passing 16384/PAGE_SIZE instead of MAX_INT work
> for your purposes?

The last arg is maxpages, I want maxbytes, no?