linux-kernel - Re: [PATCH] dax: kill uml support

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Date:   Thu, 22 Dec 2016 10:37:41 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Jan Kara <jack@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Dave Chinner <david@...morbit.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] dax: kill uml support

On Thu, Dec 22, 2016 at 8:40 AM, Ross Zwisler
<ross.zwisler@...ux.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.