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

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Date:   Thu, 22 Dec 2016 11:00:55 +0100
From:   Jan Kara <jack@...e.cz>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     Jan Kara <jack@...e.cz>, Dan Williams <dan.j.williams@...el.com>,
        akpm@...ux-foundation.org, Matthew Wilcox <mawilcox@...rosoft.com>,
        Dave Chinner <david@...morbit.com>,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] dax: kill uml support

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.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR