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

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Date:   Tue, 27 Jun 2023 18:46:01 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     Sumitra Sharma <sumitraartsy@...il.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 Tue, Jun 27, 2023 at 04:34:51PM +0200, Hans de Goede wrote:
> Hi,
> 
> On 6/27/23 15:51, Sumitra Sharma wrote:
> > kmap() has been deprecated in favor of the kmap_local_page() due to high
> > cost, restricted mapping space, the overhead of a global lock for
> > synchronization, and making the process sleep in the absence of free
> > slots.
> > 
> > kmap_local_{page, folio}() is faster than kmap() and offers thread-local
> > and CPU-local mappings, can take pagefaults in a local kmap region and
> > preserves preemption by saving the mappings of outgoing tasks and
> > restoring those of the incoming one during a context switch.
> > 
> > The difference between kmap_local_page() and kmap_local_folio() consist
> > only in the first taking a pointer to a page and the second taking two
> > arguments, a pointer to a folio and the byte offset within the folio which
> > identifies the page.
> > 
> > The mappings are kept thread local in the functions 'vboxsf_read_folio',
> > 'vboxsf_writepage', 'vboxsf_write_end' in file.c
> > 
> > Suggested-by: Ira Weiny <ira.weiny@...el.com>
> > Signed-off-by: Sumitra Sharma <sumitraartsy@...il.com>
> 
> Thanks, patch looks good to me:

It doesn't look great to me, tbh.  It's generally an antipattern to map
the page/folio up at the top and then pass the virtual address down to
the bottom.  Usually we want to work in terms of physical addresses
as long as possible.  I see the vmmdev_hgcm_function_parameter can
take physical addresses; does it work to simply use the phys_addr
instead of the linear_addr?  I see this commentary:

       /** Deprecated Doesn't work, use PAGELIST. */
        VMMDEV_HGCM_PARM_TYPE_PHYSADDR           = 3,

so, um, can we use
        /** Physical addresses of locked pages for a buffer. */
        VMMDEV_HGCM_PARM_TYPE_PAGELIST           = 10,

and convert vboxsf_read_folio() to pass the folio down to vboxsf_read()
which converts it to a PAGELIST (however one does that)?