linux-kernel - Re: [PATCH v3 4/4] fs/sysv: Replace kmap() with kmap_local

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Date:   Fri, 20 Jan 2023 05:07:48 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH v3 4/4] fs/sysv: Replace kmap() with kmap_local_page()

On Fri, Jan 20, 2023 at 04:54:51AM +0000, Matthew Wilcox wrote:

> > Sure, but... there's also this:
> > 
> > static inline void __kunmap_local(const void *addr)
> > {
> > #ifdef ARCH_HAS_FLUSH_ON_KUNMAP
> >         kunmap_flush_on_unmap(addr);
> > #endif
> > }
> > 
> > Are you sure that the guts of that thing will be happy with address that is not
> > page-aligned?  I've looked there at some point, got scared of parisc (IIRC)
> > MMU details and decided not to rely upon that...
> 
> Ugh, PA-RISC (the only implementor) definitely will flush the wrong
> addresses.  I think we should do this, as having bugs that only manifest
> on one not-well-tested architecture seems Bad.
> 
>  static inline void __kunmap_local(const void *addr)
>  {
>  #ifdef ARCH_HAS_FLUSH_ON_KUNMAP
> -       kunmap_flush_on_unmap(addr);
> +       kunmap_flush_on_unmap(PAGE_ALIGN_DOWN(addr));
>  #endif
>  }

PTR_ALIGN_DOWN(addr, PAGE_SIZE), perhaps?