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From: Dan Williams <dan.j.williams@intel.com>
To: Ross Zwisler <ross.zwisler@linux.intel.com>,
	Jan Kara <jack@suse.cz>, Dan Williams <dan.j.williams@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Matthew Wilcox <mawilcox@microsoft.com>,
	Dave Chinner <david@fromorbit.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Dave Hansen <dave.hansen@intel.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH] dax: kill uml support
Date: Thu, 22 Dec 2016 10:37:41 -0800	[thread overview]
Message-ID: <CAPcyv4iWOsfYnBUWgqZ55-6-RURo9pNLkgiB1Gu2ND=pPq2Z+g@mail.gmail.com> (raw)
In-Reply-To: <20161222164042.GA18407@linux.intel.com>

On Thu, Dec 22, 2016 at 8:40 AM, Ross Zwisler
<ross.zwisler@linux.intel.com> wrote:
> On Thu, Dec 22, 2016 at 11:00:55AM +0100, Jan Kara wrote:
>> On Wed 21-12-16 10:51:18, Ross Zwisler wrote:
>> > On Wed, Dec 21, 2016 at 09:53:47AM +0100, Jan Kara wrote:
>> > > On Tue 20-12-16 17:37:40, Dan Williams wrote:
>> > > > The lack of common transparent-huge-page helpers for UML is becoming
>> > > > increasingly painful for fs/dax.c now that it is growing more pmd
>> > > > functionality. Add UML to the list of unsupported architectures, and
>> > > > clean up no-longer-necessary ifdef as a result.
>> > > ...
>> > > > diff --git a/fs/dax.c b/fs/dax.c
>> > > > index ddcddfeaa03b..86df835783ea 100644
>> > > > --- a/fs/dax.c
>> > > > +++ b/fs/dax.c
>> > > > @@ -710,8 +710,7 @@ static void dax_mapping_entry_mkclean(struct address_space *mapping,
>> > > >                 if (follow_pte_pmd(vma->vm_mm, address, &ptep, &pmdp, &ptl))
>> > > >                         continue;
>> > > >
>> > > > -               if (pmdp) {
>> > > > -#ifdef CONFIG_FS_DAX_PMD
>> > > > +               if (pmdp && IS_ENABLED(CONFIG_FS_DAX_PMD)) {
>> > > >                         pmd_t pmd;
>> > >
>> > > So I was under the impression that pmdp can never be != NULL when
>> > > CONFIG_FS_DAX_PMD is disabled. Otherwise Ross' patch would leave ptl locked
>> > > in that case... Did I miss something or we can just remove IS_ENABLED()
>> > > check?
>> >
>> > We need the IS_ENABLED() check to prevent a different build error.
>> >
>> > The #ifdef was there to prevent compile time errors where pmd_pfn(),
>> > pmd_write(), etc were all undefined symbols because they aren't defined in
>> > the arch/um headers.
>> >
>> > For x86_64 we can get linker errors if CONFIG_TRANSPARENT_HUGEPAGE isn't set:
>> >
>> > fs/built-in.o: In function `dax_writeback_mapping_range.part.27':
>> > dax.c:(.text+0x4ce28): undefined reference to `pmdp_huge_clear_flush'
>> >
>> > In this config we do have a prototype for pmdp_huge_clear_flush() in
>> > include/asm-generic/pgtable.h so it doesn't show up as an undefined symbol,
>> > but the implementation in mm/pgtable-generic.c is in a
>> > #ifdef CONFIG_TRANSPARENT_HUGEPAGE block.
>> >
>> > The IS_ENABLED() lets the compiler optimize out the code that calls
>> > pmdp_huge_clear_flush() so it doesn't try and resolve that symbol at link
>> > time.
>>
>> Ah, OK. Won't it be cleaner to just have empty stub for
>> pmdp_huge_clear_flush() if !CONFIG_TRANSPARENT_HUGEPAGE? I like that more
>> but I can live with IS_ENABLED check as well but please add a comment that
>> it is there only to make compiler happy.
>
> Yea, making an empty stub is cleaner.  That has the added bonus that we don't
> have to rely on the assumption that 'pmdp' will always be NULL in if
> CONFIG_TRANSPARENT_HUGEPAGE isn't set.
>
> I'll reflow & send out v2.

If you're going deeper might as well look at killing the other ifdef
CONFIG_FS_DAX_PMD in that file as well.

  reply	other threads:[~2016-12-22 18:37 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-12-21  1:37 Dan Williams
2016-12-21  8:53 ` Jan Kara
2016-12-21 17:51   ` Ross Zwisler
2016-12-22 10:00     ` Jan Kara
2016-12-22 16:40       ` Ross Zwisler
2016-12-22 18:37         ` Dan Williams [this message]
2016-12-21 17:53 ` Ross Zwisler
2016-12-21 18:45   ` Ross Zwisler
2016-12-21 18:45   ` Dan Williams

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