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

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Message-ID: <cbc597b2-52a4-4c29-b240-427d353eb442@fastmail.fm>
Date: Wed, 2 Apr 2025 13:59:54 +0200
From: Bernd Schubert <bernd.schubert@...tmail.fm>
To: Jaco Kroon <jaco@....co.za>, 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.



On 4/2/25 13:13, Jaco Kroon wrote:
> 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?

I was just writing what vmalloc does - neither of its disadvantages will
have an issue here