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From: Matthew Wilcox <willy@infradead.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	linux-nvdimm@lists.01.org
Subject: Re: [PATCH 1/4] mm: Introduce and use page_cache_empty
Date: Sun, 16 Aug 2020 02:48:51 +0100	[thread overview]
Message-ID: <20200816014851.GE17456@casper.infradead.org> (raw)
In-Reply-To: <20200806232400.s7nhmnoi3tkk7p2r@box>

On Fri, Aug 07, 2020 at 02:24:00AM +0300, Kirill A. Shutemov wrote:
> On Tue, Aug 04, 2020 at 05:17:52PM +0100, Matthew Wilcox (Oracle) wrote:
> > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> > index 484a36185bb5..a474a92a2a72 100644
> > --- a/include/linux/pagemap.h
> > +++ b/include/linux/pagemap.h
> > @@ -18,6 +18,11 @@
> >  
> >  struct pagevec;
> >  
> > +static inline bool page_cache_empty(struct address_space *mapping)
> > +{
> > +	return xa_empty(&mapping->i_pages);
> 
> What about something like
> 
> 	bool empty = xa_empty(&mapping->i_pages);
> 	VM_BUG_ON(empty && mapping->nrpages);
> 	return empty;

I tried this and it's triggered by generic/418.  The problem
is that it's called when the pagecache lock isn't held (by
invalidate_inode_pages2_range), so it's possible for xa_empty() to
return true, then a page be added to the page cache, and mapping->pages
be incremented to 1.  That seems to be what's happened here:

(gdb) p/x *(struct address_space *)0xffff88804b21b360
$2 = {host = 0xffff88804b21b200, i_pages = {xa_lock = {{rlock = {raw_lock = {{
              val = {counter = 0x0}, {locked = 0x0, pending = 0x0}, {
                locked_pending = 0x0, tail = 0x0}}}}}}, xa_flags = 0x21, 
*  xa_head = 0xffffea0001e187c0}, gfp_mask = 0x100c4a, i_mmap_writable = {
    counter = 0x0}, nr_thps = {counter = 0x0}, i_mmap = {rb_root = {
      rb_node = 0x0}, rb_leftmost = 0x0}, i_mmap_rwsem = {count = {
      counter = 0x0}, owner = {counter = 0x0}, osq = {tail = {counter = 0x0}}, 
    wait_lock = {raw_lock = {{val = {counter = 0x0}, {locked = 0x0, 
            pending = 0x0}, {locked_pending = 0x0, tail = 0x0}}}}, 
    wait_list = {next = 0xffff88804b21b3b0, prev = 0xffff88804b21b3b0}}, 
* nrpages = 0x1, writeback_index = 0x0, a_ops = 0xffffffff81c2ed60, 
  flags = 0x40, wb_err = 0x0, private_lock = {{rlock = {raw_lock = {{val = {
              counter = 0x0}, {locked = 0x0, pending = 0x0}, {
              locked_pending = 0x0, tail = 0x0}}}}}}, private_list = {
    next = 0xffff88804b21b3e8, prev = 0xffff88804b21b3e8}, private_data = 0x0}

(marked the critical lines with *)

  reply	other threads:[~2020-08-16  1:48 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-04 16:17 [PATCH 0/4] Remove nrexceptional tracking Matthew Wilcox (Oracle)
2020-08-04 16:17 ` [PATCH 1/4] mm: Introduce and use page_cache_empty Matthew Wilcox (Oracle)
2020-08-06 23:24   ` Kirill A. Shutemov
2020-08-16  1:48     ` Matthew Wilcox [this message]
2020-08-04 16:17 ` [PATCH 2/4] mm: Stop accounting shadow entries Matthew Wilcox (Oracle)
2020-08-04 16:17 ` [PATCH 3/4] dax: Account DAX entries as nrpages Matthew Wilcox (Oracle)
2020-08-04 16:17 ` [PATCH 4/4] mm: Remove nrexceptional from inode Matthew Wilcox (Oracle)
2020-08-06 19:44 ` [PATCH 0/4] Remove nrexceptional tracking Verma, Vishal L
2020-08-06 20:16   ` Verma, Vishal L
2020-10-08 19:33     ` Matthew Wilcox
2020-10-09 23:04       ` Verma, Vishal L

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