linux-kernel - Re: [RFC PATCH v2 0/1] Introduce vmap

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Message-ID: <Z-tGXf5mfTclxZgl@fedora>
Date: Mon, 31 Mar 2025 18:50:21 -0700
From: "Vishal Moola (Oracle)" <vishal.moola@...il.com>
To: Huan Yang <link@...o.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Brendan Jackman <jackmanb@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 0/1] Introduce vmap_file()

On Mon, Mar 31, 2025 at 10:05:53AM +0800, Huan Yang wrote:
> HI Vishal,
> 
> 在 2025/3/29 05:13, Vishal Moola (Oracle) 写道:
> > Currently, users have to call vmap() or vmap_pfn() to map pages to
> > kernel virtual space. vmap_pfn() is for special pages (i.e. pfns
> > without struct page). vmap() handles normal pages.
> > 
> > With large folios, we may want to map ranges that only span
> > part of a folio (i.e. mapping half of a 2Mb folio).
> > vmap_file() will allow us to do so.
> 
> You mention vmap_file can support range folio vmap, but when I look code, I can't figure out
> 
> how to use, maybe I missed something? :)

I took a look at the udma-buf code. Rather than iterating through the
folios using pfns, you can calculate the corresponding file offsets 
(maybe you already have them?) to map the desired folios.

> And this API still aim to file vmap, Maybe not suitable for the problem I mentioned in:
> 
> https://lore.kernel.org/lkml/20250312061513.1126496-1-link@vivo.com/

I'm not sure which problem you're referring to, could you be more
specific?

> Thanks,
> Huan Yang
> 
> > Create a function, vmap_file(), to map a specified range of a given
> > file to kernel virtual space. vmap_file() is an in-kernel equivalent
> > to mmap(), and can be useful for filesystems.
> > 
> > ---
> > v2:
> >    - Reword cover letter to provide a clearer overview of the current
> >    vmalloc APIs, and usefulness of vmap_file()
> >    - EXPORT_SYMBOL_GPL() instead of EXPORT_SYMBOL()
> >    - Provide support to partially map file folios
> >    - Demote this to RFC while we look for users
> > --
> > I don't have a user for this function right now, but it will be
> > useful as users start converting to using large folios. I'm just
> > putting it out here for anyone that may find a use for it.
> > 
> > This seems like the sensible way to implement it, but I'm open
> > to tweaking the functions semantics.
> > 
> > I've Cc-ed a couple people that mentioned they might be interested
> > in using it.
> > 
> > Vishal Moola (Oracle) (1):
> >    mm/vmalloc: Introduce vmap_file()
> > 
> >   include/linux/vmalloc.h |   2 +
> >   mm/vmalloc.c            | 113 ++++++++++++++++++++++++++++++++++++++++
> >   2 files changed, 115 insertions(+)
> >