linux-kernel - Re: [PATCH 2/2] vfree: Update documentation

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Message-ID: <20200922164240.GJ32101@casper.infradead.org>
Date:   Tue, 22 Sep 2020 17:42:40 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Uladzislau Rezki <urezki@...il.com>
Subject: Re: [PATCH 2/2] vfree: Update documentation

On Tue, Sep 22, 2020 at 05:31:36PM +0200, Christoph Hellwig wrote:
> On Tue, Sep 22, 2020 at 04:22:40PM +0100, Matthew Wilcox wrote:
> > On Tue, Sep 22, 2020 at 05:09:10PM +0200, Christoph Hellwig wrote:
> > > On Tue, Sep 22, 2020 at 04:06:03PM +0100, Matthew Wilcox wrote:
> > > > I don't think it makes sense to list all vmalloc-style allocators here.
> > > > It won't be updated by people who add new variations.  How about this?
> > > > 
> > > >  * Free the virtually continuous memory area starting at @addr, as
> > > >  * obtained from one of the vmalloc() family of APIs.  This will
> > > >  * usually also free the physical memory underlying the virtual
> > > >  * allocation, but that memory is reference counted, so it will not
> > > >  * be freed until the last user goes away.
> > > >  *
> > > >  * If @addr is NULL, no operation is performed.
> > > > 
> > > > I'm trying to strike a balance between being accurate and not requiring
> > > > device driver authors to learn all about struct page.  I may be too
> > > > close to the implementation to write good documentation for it.
> > > 
> > > I think the above is sensible, but not enough.  vmap really needs to
> > > be treated special, as by default area->pages for vmap is NULL.  So
> > > for vfree to be useful on a vmap mapping, the callers needs to
> > > manually set it up by poking into the internals.  Actually, I think
> > > we really want another API rather than vmap for that.  Let me respin
> > > my series to include that.
> > 
> > I've been thinking about somethng like:
> > 
> > void *vmap_mapping(struct address_space *mapping, pgoff_t start,
> > 		unsigned long len);
> > 
> > but it doesn't quite work for the shmem cases because they need to use
> > shmem_read_mapping_page_gfp() instead of ->readpage.  I'd also want it
> > to work for DAX, but I don't have a user for that yet so it's hard to
> > justify adding it.
> 
> That seems a little too special cased to me.

It's for filesystems that currently use map_bh or kmap_atomic() to
get at their metadata (directories, superblocks, etc).

> FYI, if you are fine with it I'll pick your two patches with the
> updates from this thread up for my series.  It has be now morphed into
> a general vmalloc.c cleanup series.

Yes, that's absolutely fine.  Thanks!