Date:   Thu, 14 Jan 2021 09:20:18 -0800
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     zhong jiang <zhongjiang-ali@...ux.alibaba.com>
Cc:     Ruan Shiyang <ruansy.fnst@...fujitsu.com>, Jan Kara <jack@...e.cz>,
        linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-raid@...r.kernel.org,
        darrick.wong@...cle.com, dan.j.williams@...el.com,
        david@...morbit.com, hch@....de, song@...nel.org, rgoldwyn@...e.de,
        qi.fuli@...itsu.com, y-goto@...itsu.com
Subject: Re: [PATCH 04/10] mm, fsdax: Refactor memory-failure handler for dax
 mapping

On Thu, Jan 14, 2021 at 05:38:33PM +0800, zhong jiang wrote:
> 
> On 2021/1/14 11:52 上午, Ruan Shiyang wrote:
> > 
> > 
> > On 2021/1/14 上午11:26, zhong jiang wrote:
> > > 
> > > On 2021/1/14 9:44 上午, Ruan Shiyang wrote:
> > > > 
> > > > 
> > > > On 2021/1/13 下午6:04, zhong jiang wrote:
> > > > > 
> > > > > On 2021/1/12 10:55 上午, Ruan Shiyang wrote:
> > > > > > 
> > > > > > 
> > > > > > On 2021/1/6 下午11:41, Jan Kara wrote:
> > > > > > > On Thu 31-12-20 00:55:55, Shiyang Ruan wrote:
> > > > > > > > The current memory_failure_dev_pagemap() can
> > > > > > > > only handle single-mapped
> > > > > > > > dax page for fsdax mode.  The dax page could be
> > > > > > > > mapped by multiple files
> > > > > > > > and offsets if we let reflink feature & fsdax
> > > > > > > > mode work together. So,
> > > > > > > > we refactor current implementation to support
> > > > > > > > handle memory failure on
> > > > > > > > each file and offset.
> > > > > > > > 
> > > > > > > > Signed-off-by: Shiyang Ruan <ruansy.fnst@...fujitsu.com>
> > > > > > > 
> > > > > > > Overall this looks OK to me, a few comments below.
> > > > > > > 
> > > > > > > > ---
> > > > > > > >   fs/dax.c            | 21 +++++++++++
> > > > > > > >   include/linux/dax.h |  1 +
> > > > > > > >   include/linux/mm.h  |  9 +++++
> > > > > > > >   mm/memory-failure.c | 91
> > > > > > > > ++++++++++++++++++++++++++++++++++-----------
> > > > > > > >   4 files changed, 100 insertions(+), 22 deletions(-)
> > > > > > 
> > > > > > ...
> > > > > > 
> > > > > > > >   @@ -345,9 +348,12 @@ static void
> > > > > > > > add_to_kill(struct task_struct *tsk, struct page
> > > > > > > > *p,
> > > > > > > >       }
> > > > > > > >         tk->addr = page_address_in_vma(p, vma);
> > > > > > > > -    if (is_zone_device_page(p))
> > > > > > > > -        tk->size_shift = dev_pagemap_mapping_shift(p, vma);
> > > > > > > > -    else
> > > > > > > > +    if (is_zone_device_page(p)) {
> > > > > > > > +        if (is_device_fsdax_page(p))
> > > > > > > > +            tk->addr = vma->vm_start +
> > > > > > > > +                    ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
> > > > > > > 
> > > > > > > It seems strange to use 'pgoff' for dax pages and
> > > > > > > not for any other page.
> > > > > > > Why? I'd rather pass correct pgoff from all callers
> > > > > > > of add_to_kill() and
> > > > > > > avoid this special casing...
> > > > > > 
> > > > > > Because one fsdax page can be shared by multiple pgoffs.
> > > > > > I have to pass each pgoff in each iteration to calculate
> > > > > > the address in vma (for tk->addr).  Other kinds of pages
> > > > > > don't need this. They can get their unique address by
> > > > > > calling "page_address_in_vma()".
> > > > > > 
> > > > > IMO,   an fsdax page can be shared by multiple files rather
> > > > > than multiple pgoffs if fs query support reflink.   Because
> > > > > an page only located in an mapping(page->mapping is
> > > > > exclusive), hence it  only has an pgoff or index pointing at
> > > > > the node.
> > > > > 
> > > > >   or  I miss something for the feature ?  thanks,
> > > > 
> > > > Yes, a fsdax page is shared by multiple files because of
> > > > reflink. I think my description of 'pgoff' here is not correct. 
> > > > This 'pgoff' means the offset within the a file. (We use rmap to
> > > > find out all the sharing files and their offsets.)  So, I said
> > > > that "can be shared by multiple pgoffs".  It's my bad.
> > > > 
> > > > I think I should name it another word to avoid misunderstandings.
> > > > 
> > > IMO,  All the sharing files should be the same offset to share the
> > > fsdax page.  why not that ?
> > 
> > The dedupe operation can let different files share their same data
> > extent, though offsets are not same.  So, files can share one fsdax page
> > at different offset.
> Ok,  Get it.
> > 
> > > As you has said,  a shared fadax page should be inserted to
> > > different mapping files.  but page->index and page->mapping is
> > > exclusive.  hence an page only should be placed in an mapping tree.
> > 
> > We can't use page->mapping and page->index here for reflink & fsdax. And
> > that's this patchset aims to solve.  I introduced a series of
> > ->corrupted_range(), from mm to pmem driver to block device and finally
> > to filesystem, to use rmap feature of filesystem to find out all files
> > sharing same data extent (fsdax page).
> 
> From this patch,  each file has mapping tree,  the shared page will be
> inserted into multiple file mapping tree.  then filesystem use file and
> offset to get the killed process.   Is it correct?

FWIW I thought the purpose of this patchset is to remove the (dax)
memory poison code's reliance on the pagecache mapping structure by
pushing poison notifications directly into the filesystem and letting
the filesystem perform reverse lookup operations to figure out which
file(s) have gone bad, and using the file list to call back into the mm
to kill processes.

Once that's done, I think(?) that puts us significantly closer to being
able to share pmem between files in dax mode without having to rewrite
the entire memory manager's mapping and rmapping code to support
sharing.

--D

> Thanks,
> 
> > 
> > 
> > -- 
> > Thanks,
> > Ruan Shiyang.
> > 
> > > 
> > > And In the current patch,  we failed to found out that all process
> > > use the fsdax page shared by multiple files and kill them.
> > > 
> > > 
> > > Thanks,
> > > 
> > > > -- 
> > > > Thanks,
> > > > Ruan Shiyang.
> > > > 
> > > > > 
> > > > > > So, I added this fsdax case here. This patchset only
> > > > > > implemented the fsdax case, other cases also need to be
> > > > > > added here if to be implemented.
> > > > > > 
> > > > > > 
> > > > > > -- 
> > > > > > Thanks,
> > > > > > Ruan Shiyang.
> > > > > > 
> > > > > > > 
> > > > > > > > +        tk->size_shift =
> > > > > > > > dev_pagemap_mapping_shift(p, vma, tk->addr);
> > > > > > > > +    } else
> > > > > > > >           tk->size_shift = page_shift(compound_head(p));
> > > > > > > >         /*
> > > > > > > > @@ -495,7 +501,7 @@ static void
> > > > > > > > collect_procs_anon(struct page *page, struct
> > > > > > > > list_head *to_kill,
> > > > > > > >               if (!page_mapped_in_vma(page, vma))
> > > > > > > >                   continue;
> > > > > > > >               if (vma->vm_mm == t->mm)
> > > > > > > > -                add_to_kill(t, page, vma, to_kill);
> > > > > > > > +                add_to_kill(t, page, NULL, 0, vma, to_kill);
> > > > > > > >           }
> > > > > > > >       }
> > > > > > > >       read_unlock(&tasklist_lock);
> > > > > > > > @@ -505,24 +511,19 @@ static void
> > > > > > > > collect_procs_anon(struct page *page, struct
> > > > > > > > list_head *to_kill,
> > > > > > > >   /*
> > > > > > > >    * Collect processes when the error hit a file mapped page.
> > > > > > > >    */
> > > > > > > > -static void collect_procs_file(struct page
> > > > > > > > *page, struct list_head *to_kill,
> > > > > > > > -                int force_early)
> > > > > > > > +static void collect_procs_file(struct page
> > > > > > > > *page, struct address_space *mapping,
> > > > > > > > +        pgoff_t pgoff, struct list_head *to_kill, int force_early)
> > > > > > > >   {
> > > > > > > >       struct vm_area_struct *vma;
> > > > > > > >       struct task_struct *tsk;
> > > > > > > > -    struct address_space *mapping = page->mapping;
> > > > > > > > -    pgoff_t pgoff;
> > > > > > > >         i_mmap_lock_read(mapping);
> > > > > > > >       read_lock(&tasklist_lock);
> > > > > > > > -    pgoff = page_to_pgoff(page);
> > > > > > > >       for_each_process(tsk) {
> > > > > > > >           struct task_struct *t =
> > > > > > > > task_early_kill(tsk, force_early);
> > > > > > > > -
> > > > > > > >           if (!t)
> > > > > > > >               continue;
> > > > > > > > -        vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff,
> > > > > > > > -                      pgoff) {
> > > > > > > > +        vma_interval_tree_foreach(vma,
> > > > > > > > &mapping->i_mmap, pgoff, pgoff) {
> > > > > > > >               /*
> > > > > > > >                * Send early kill signal to tasks where a vma covers
> > > > > > > >                * the page but the corrupted page is not necessarily
> > > > > > > > @@ -531,7 +532,7 @@ static void
> > > > > > > > collect_procs_file(struct page *page, struct
> > > > > > > > list_head *to_kill,
> > > > > > > >                * to be informed of all such data corruptions.
> > > > > > > >                */
> > > > > > > >               if (vma->vm_mm == t->mm)
> > > > > > > > -                add_to_kill(t, page, vma, to_kill);
> > > > > > > > +                add_to_kill(t, page, mapping,
> > > > > > > > pgoff, vma, to_kill);
> > > > > > > >           }
> > > > > > > >       }
> > > > > > > >       read_unlock(&tasklist_lock);
> > > > > > > > @@ -550,7 +551,8 @@ static void
> > > > > > > > collect_procs(struct page *page, struct
> > > > > > > > list_head *tokill,
> > > > > > > >       if (PageAnon(page))
> > > > > > > >           collect_procs_anon(page, tokill, force_early);
> > > > > > > >       else
> > > > > > > > -        collect_procs_file(page, tokill, force_early);
> > > > > > > > +        collect_procs_file(page, page->mapping,
> > > > > > > > page_to_pgoff(page),
> > > > > > > 
> > > > > > > Why not use page_mapping() helper here? It would be
> > > > > > > safer for THPs if they
> > > > > > > ever get here...
> > > > > > > 
> > > > > > >                                 Honza
> > > > > > > 
> > > > > > 
> > > > > 
> > > > > 
> > > > 
> > > 
> > > 
> > 

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