linux-kernel - Re: [PATCH] fs/vboxsf: Replace kmap() with kmap_local

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Message-ID: <ZJ2Yb8YOpakO7SbY@casper.infradead.org>
Date:   Thu, 29 Jun 2023 15:42:55 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Sumitra Sharma <sumitraartsy@...il.com>
Cc:     Hans de Goede <hdegoede@...hat.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ira Weiny <ira.weiny@...el.com>,
        Fabio <fmdefrancesco@...il.com>, Deepak R Varma <drv@...lo.com>
Subject: Re: [PATCH] fs/vboxsf: Replace kmap() with kmap_local_{page, folio}()

On Thu, Jun 29, 2023 at 02:28:44AM -0700, Sumitra Sharma wrote:
> On Wed, Jun 28, 2023 at 06:15:04PM +0100, Matthew Wilcox wrote:
> > Here's a more comprehensive read_folio patch.  It's not at all
> > efficient, but then if we wanted an efficient vboxsf, we'd implement
> > vboxsf_readahead() and actually do an async call with deferred setting
> > of the uptodate flag.  I can consult with anyone who wants to do all
> > this work.
>
> So, after reading the comments, I understood that the problem presented 
> by Hans and Matthew is as follows:
> 
> 1) In the current code, the buffers used by vboxsf_write()/vboxsf_read() are 
> translated to PAGELIST-s before passing to the hypervisor, 
> but inefficiently— it first maps a page in vboxsf_read_folio() and then 
> calls page_to_phys(virt_to_page()) in the function hgcm_call_init_linaddr(). 

It does ... and I'm not even sure that virt_to_page() works for kmapped
pages.  Has it been tested with a 32-bit guest with, say, 4-8GB of memory?

> The inefficiency in the current implementation arises due to the unnecessary 
> mapping of a page in vboxsf_read_folio() because the mapping output, i.e. the 
> linear address, is used deep down in file 'drivers/virt/vboxguest/vboxguest_utils.c'. 
> Hence, the mapping must be done in this file; to do so, the folio must be passed 
> until this point. It can be done by adding a new member, 'struct folio *folio', 
> in the 'struct vmmdev_hgcm_function_parameter64'. 

That's not the way to do it (as Hans already said).

The other problem is that vboxsf_read() is synchronous.  It makes the
call to the host, then waits for the outcome.  What we really need is
a vboxsf_readahead() that looks something like this:

static void vboxsf_readahead(struct readahead_control *ractl)
{
	unsigned int nr = readahead_count(ractl);
	req = vbg_req_alloc(... something involving nr ...);
	... fill in the page array ...
	... submit the request ...
}

You also need to set up a kthread that will sit on the hgcm_wq and handle
the completions that come in (where you'd call folio_mark_uptodate() if
the call is successful, folio_unlock() to indicate the I/O has completed,
etc, etc).

Then go back to read_folio() (which can be synchronous), and maybe factor
out the parts of vboxsf_readahead() that can be reused for filling in
the vbg_req.

Hans might well have better ideas about this could be structured; I'm
new to the vbox code.