linux-kernel - Re: [PATCH 2/2] fuse: Adjust readdir() buffer to requesting buffer size.

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Message-ID: <f22c14e1-43d9-4976-b13e-a664f5195233@uls.co.za>
Date: Wed, 2 Apr 2025 13:13:09 +0200
From: Jaco Kroon <jaco@....co.za>
To: Bernd Schubert <bernd.schubert@...tmail.fm>,
 Miklos Szeredi <miklos@...redi.hu>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 christophe.jaillet@...adoo.fr, joannelkoong@...il.com,
 rdunlap@...radead.org, trapexit@...wn.link, david.laight.linux@...il.com
Subject: Re: [PATCH 2/2] fuse: Adjust readdir() buffer to requesting buffer
 size.

Hi,

On 2025/04/02 11:10, Bernd Schubert wrote:
>
> On 4/2/25 10:52, Jaco Kroon wrote:
>> Hi,
>>
>> On 2025/04/02 10:18, Miklos Szeredi wrote:
>>> On Wed, 2 Apr 2025 at 09:55, Jaco Kroon <jaco@....co.za> wrote:
>>>> Hi,
>>>>
>>>> I can definitely build on that, thank you.
>>>>
>>>> What's the advantage of kvmalloc over folio's here, why should it be
>>>> preferred?
>>> It offers the best of both worlds: first tries plain malloc (which
>>> just does a folio alloc internally for size > PAGE_SIZE) and if that
>>> fails, falls back to vmalloc, which should always succeed since it
>>> uses order 0 pages.
>> So basically assigns the space, but doesn't commit physical pages for
>> the allocation, meaning first access will cause a page fault, and single
>> page allocation at that point in time?  Or is it merely the fact that
>> vmalloc may return a virtual contiguous block that's not physically
>> contiguous?
>
> Yes vmalloc return buffers might not be physically contiguous - not
> suitable for hardware DMA. And AFAIK it is also a blocking allocation.
How do I go about confirming?  Can that behaviour be stopped so that in 
the case where it would block we can return an EAGAIN or EWOULDBLOCK 
error code instead?  Is that even desired?

Don't think hardware DMA is an issue here, so that's at least not an 
issue, but the blocking might be?

Kind regards,
Jaco