linux-fsdevel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Jan Kara <jack@suse.cz>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Jan Kara <jack@suse.cz>, linux-nvdimm <linux-nvdimm@lists.01.org>,
	Christoph Hellwig <hch@lst.de>,
	Matthew Wilcox <mawilcox@microsoft.com>,
	Ross Zwisler <ross.zwisler@linux.intel.com>,
	Linux MM <linux-mm@kvack.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	"Luck, Tony" <tony.luck@intel.com>,
	linux-xfs <linux-xfs@vger.kernel.org>,
	"Darrick J. Wong" <darrick.wong@oracle.com>
Subject: Re: [PATCH 05/11] filesystem-dax: set page->index
Date: Thu, 31 May 2018 12:08:49 +0200	[thread overview]
Message-ID: <20180531100849.xkwe5xsjpkw6nev7@quack2.suse.cz> (raw)
In-Reply-To: <CAPcyv4iPa_n7c6iLRtNyE4GdXcn7JcF=Z1bUDmNrjrKvnLic2A@mail.gmail.com>

On Wed 30-05-18 16:21:33, Dan Williams wrote:
> On Wed, May 30, 2018 at 1:13 AM, Jan Kara <jack@suse.cz> wrote:
> > On Tue 29-05-18 18:38:41, Dan Williams wrote:
> >> On Wed, May 23, 2018 at 1:40 AM, Jan Kara <jack@suse.cz> wrote:
> >> > On Tue 22-05-18 07:39:57, Dan Williams wrote:
> >> >> In support of enabling memory_failure() handling for filesystem-dax
> >> >> mappings, set ->index to the pgoff of the page. The rmap implementation
> >> >> requires ->index to bound the search through the vma interval tree. The
> >> >> index is set and cleared at dax_associate_entry() and
> >> >> dax_disassociate_entry() time respectively.
> >> >>
> >> >> Cc: Jan Kara <jack@suse.cz>
> >> >> Cc: Christoph Hellwig <hch@lst.de>
> >> >> Cc: Matthew Wilcox <mawilcox@microsoft.com>
> >> >> Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
> >> >> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> >> >> ---
> >> >>  fs/dax.c |   11 ++++++++---
> >> >>  1 file changed, 8 insertions(+), 3 deletions(-)
> >> >>
> >> >> diff --git a/fs/dax.c b/fs/dax.c
> >> >> index aaec72ded1b6..2e4682cd7c69 100644
> >> >> --- a/fs/dax.c
> >> >> +++ b/fs/dax.c
> >> >> @@ -319,18 +319,22 @@ static unsigned long dax_radix_end_pfn(void *entry)
> >> >>       for (pfn = dax_radix_pfn(entry); \
> >> >>                       pfn < dax_radix_end_pfn(entry); pfn++)
> >> >>
> >> >> -static void dax_associate_entry(void *entry, struct address_space *mapping)
> >> >> +static void dax_associate_entry(void *entry, struct address_space *mapping,
> >> >> +             struct vm_area_struct *vma, unsigned long address)
> >> >>  {
> >> >> -     unsigned long pfn;
> >> >> +     unsigned long size = dax_entry_size(entry), pfn, index;
> >> >> +     int i = 0;
> >> >>
> >> >>       if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
> >> >>               return;
> >> >>
> >> >> +     index = linear_page_index(vma, address & ~(size - 1));
> >> >>       for_each_mapped_pfn(entry, pfn) {
> >> >>               struct page *page = pfn_to_page(pfn);
> >> >>
> >> >>               WARN_ON_ONCE(page->mapping);
> >> >>               page->mapping = mapping;
> >> >> +             page->index = index + i++;
> >> >>       }
> >> >>  }
> >> >
> >> > Hum, this just made me think: How is this going to work with XFS reflink?
> >> > In fact is not the page->mapping association already broken by XFS reflink?
> >> > Because with reflink we can have two or more mappings pointing to the same
> >> > physical blocks (i.e., pages in DAX case)...
> >>
> >> Good question. I assume we are ok in the non-DAX reflink case because
> >> rmap of failing / poison pages is only relative to the specific page
> >> cache page for a given inode in the reflink. However, DAX would seem
> >> to break this because we only get one shared 'struct page' for all
> >> possible mappings of the physical file block. I think this means for
> >> iterating over the rmap of "where is this page mapped" would require
> >> iterating over the other "sibling" inodes that know about the given
> >> physical file block.
> >>
> >> As far as I can see reflink+dax would require teaching kernel code
> >> paths that ->mapping may not be a singular relationship. Something
> >> along the line's of what Jerome was presenting at LSF to create a
> >> special value to indicate, "call back into the filesystem (or the page
> >> owner)" to perform this operation.
> >>
> >> In the meantime the kernel crashes when userspace accesses poisoned
> >> pmem via DAX. I assume that reworking rmap for the dax+reflink case
> >> should not block dax poison handling? Yell if you disagree.
> >
> > The thing is, up until get_user_pages() vs truncate series ("fs, dax: use
> > page->mapping to warn if truncate collides with a busy page" in
> > particular), DAX was perfectly fine with reflinks since we never needed
> > page->mapping.
> 
> Sure, but if this rmap series had come first I still would have needed
> to implement ->mapping. So unless we invent a general ->mapping
> replacement and switch all mapping users, it was always going to
> collide with DAX eventually.

Yes, my comment was more in direction that life would be easier if we could
keep DAX without rmap support but I guess that's just too cumbersome.

> > Now this series adds even page->index dependency which makes
> > life for rmap with reflinks even harder. So if nothing else we should at
> > least make sure reflinked filesystems cannot be mounted with dax mount
> > option for now and seriously start looking into how to implement rmap with
> > reflinked files for DAX because this noticeably reduces its usefulness.
> 
> This restriction is already in place. In xfs_reflink_remap_range() we have:
> 
>         /* Don't share DAX file data for now. */
>         if (IS_DAX(inode_in) || IS_DAX(inode_out))
>                 goto out_unlock;

OK, good.

> All this said, perhaps we don't need to set ->link, it would just mean
> a wider search through the rmap tree to find if the given page is
> mapped. So, I think I can forgo setting ->link if I teach the rmap
> code to search the entire ->mapping.

I guess you mean ->index in the above. And now when thinking about it I don't
think it is worth the complications to avoid using ->index.

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

  reply	other threads:[~2018-05-31 10:08 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-22 14:39 [PATCH 00/11] mm: Teach memory_failure() about ZONE_DEVICE pages Dan Williams
2018-05-22 14:39 ` [PATCH 01/11] device-dax: convert to vmf_insert_mixed and vm_fault_t Dan Williams
2018-05-22 14:39 ` [PATCH 02/11] device-dax: cleanup vm_fault de-reference chains Dan Williams
2018-05-22 14:39 ` [PATCH 03/11] device-dax: enable page_mapping() Dan Williams
2018-05-23  9:03   ` Jan Kara
2018-05-30 19:54   ` kbuild test robot
2018-05-22 14:39 ` [PATCH 04/11] device-dax: set page->index Dan Williams
2018-05-22 14:39 ` [PATCH 05/11] filesystem-dax: " Dan Williams
2018-05-23  8:40   ` Jan Kara
2018-05-30  1:38     ` Dan Williams
2018-05-30  8:13       ` Jan Kara
2018-05-30 23:21         ` Dan Williams
2018-05-31 10:08           ` Jan Kara [this message]
2018-05-31 21:49             ` Dan Williams
2018-05-22 14:40 ` [PATCH 06/11] filesystem-dax: perform __dax_invalidate_mapping_entry() under the page lock Dan Williams
2018-05-23  9:35   ` Jan Kara
2018-05-23 13:50     ` Dan Williams
2018-05-22 14:40 ` [PATCH 07/11] mm, madvise_inject_error: fix page count leak Dan Williams
2018-05-23  4:19   ` Naoya Horiguchi
2018-05-24 20:55     ` Dan Williams
2018-05-22 14:40 ` [PATCH 08/11] x86, memory_failure: introduce {set, clear}_mce_nospec() Dan Williams
2018-05-22 14:40 ` [PATCH 09/11] mm, memory_failure: pass page size to kill_proc() Dan Williams
2018-05-23  6:41   ` Naoya Horiguchi
2018-05-22 14:40 ` [PATCH 10/11] mm, memory_failure: teach memory_failure() about dev_pagemap pages Dan Williams
2018-05-23  6:48   ` Naoya Horiguchi
2018-05-22 14:40 ` [PATCH 11/11] libnvdimm, pmem: restore page attributes when clearing errors Dan Williams

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20180531100849.xkwe5xsjpkw6nev7@quack2.suse.cz \
    --to=jack@suse.cz \
    --cc=dan.j.williams@intel.com \
    --cc=darrick.wong@oracle.com \
    --cc=hch@lst.de \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=linux-nvdimm@lists.01.org \
    --cc=linux-xfs@vger.kernel.org \
    --cc=mawilcox@microsoft.com \
    --cc=ross.zwisler@linux.intel.com \
    --cc=tony.luck@intel.com \
    --subject='Re: [PATCH 05/11] filesystem-dax: set page->index' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).