On Sat 17-12-16 09:04:50, Dave Chinner wrote:
> On Fri, Dec 16, 2016 at 09:19:16AM -0700, Ross Zwisler wrote:
> > On Fri, Dec 16, 2016 at 12:07:30PM +1100, Dave Chinner wrote:
> > > On Thu, Dec 15, 2016 at 04:40:41PM -0700, Dave Jiang wrote:
> > > > The caller into dax needs to clear __GFP_FS mask bit since it's
> > > > responsible for acquiring locks / transactions that blocks __GFP_FS
> > > > allocation. The caller will restore the original mask when dax function
> > > > returns.
> > >
> > > What's the allocation problem you're working around here? Can you
> > > please describe the call chain that is the problem?
> > >
> > > > xfs_ilock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> > > >
> > > > if (IS_DAX(inode)) {
> > > > + gfp_t old_gfp = vmf->gfp_mask;
> > > > +
> > > > + vmf->gfp_mask &= ~__GFP_FS;
> > > > ret = dax_iomap_fault(vma, vmf, &xfs_iomap_ops);
> > > > + vmf->gfp_mask = old_gfp;
> > >
> > > I really have to say that I hate code that clears and restores flags
> > > without any explanation of why the code needs to play flag tricks. I
> > > take one look at the XFS fault handling code and ask myself now "why
> > > the hell do we need to clear those flags?" Especially as the other
> > > paths into generic fault handlers /don't/ require us to do this.
> > > What does DAX do that require us to treat memory allocation contexts
> > > differently to the filemap_fault() path?
> >
> > This was done in response to Jan Kara's concern:
> >
> > The gfp_mask that propagates from __do_fault() or do_page_mkwrite() is fine
> > because at that point it is correct. But once we grab filesystem locks which
> > are not reclaim safe, we should update vmf->gfp_mask we pass further down
> > into DAX code to not contain __GFP_FS (that's a bug we apparently have
> > there). And inside DAX code, we definitely are not generally safe to add
> > __GFP_FS to mapping_gfp_mask(). Maybe we'd be better off propagating struct
> > vm_fault into this function, using passed gfp_mask there and make sure
> > callers update gfp_mask as appropriate.
> >
> > https://lkml.org/lkml/2016/10/4/37
> >
> > IIUC I think the concern is that, for example, in xfs_filemap_page_mkwrite()
> > we take a read lock on the struct inode.i_rwsem before we call
> > dax_iomap_fault().
>
> That, my friends, is exactly the problem that mapping_gfp_mask() is
> meant to solve. This:
>
> > > > + vmf.gfp_mask = mapping_gfp_mask(mapping) | __GFP_FS | __GFP_IO;
>
> Is just so wrong it's not funny.
You mean like in mm/memory.c: __get_fault_gfp_mask()?
Which was introduced by commit c20cd45eb017 "mm: allow GFP_{FS,IO} for
page_cache_read page cache allocation" by Michal (added to CC) and you were
even on CC ;).
The code here was replicating __get_fault_gfp_mask() and in fact the idea
of the cleanup is to get rid of this code and take whatever is in
vmf.gfp_mask and mask off __GFP_FS in the filesystem if it deems it is
needed (e.g. ext4 really needs this as inode reclaim is depending on being
able to force a transaction commit).
I agree with your point about comments, we should add those when changing
gfp_mask.
> The whole point of mapping_gfp_mask() is to remove flags from the
> gfp_mask used to do mapping+page cache related allocations that the
> mapping->host considers dangerous when the host may be holding locks.
> This includes mapping tree allocations, and anything else required
> to set up a new entry in the mapping during IO path operations. That
> includes page fault operations...
>
> e.g. in xfs_setup_inode():
>
> /*
> * Ensure all page cache allocations are done from GFP_NOFS context to
> * prevent direct reclaim recursion back into the filesystem and blowing
> * stacks or deadlocking.
> */
> gfp_mask = mapping_gfp_mask(inode->i_mapping);
> mapping_set_gfp_mask(inode->i_mapping, (gfp_mask & ~(__GFP_FS)));
>
> i.e. XFS considers it invalid to use GFP_FS at all for mapping
> allocations in the io path, because we *know* that we hold
> filesystems locks over those allocations.
Well, this is a discussion you should probably have with Michal. DAX code
was just mirroring what the generic code does. Michal had a valid points
why page fault path is special and allocation of pages for a page fault
should be fine with __GFP_FS - but if those assumptions are wrong for XFS,
generic code needs to be fixed.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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