[bug] userspace hitting sporadic SIGBUS on xfs (Power9, ppc64le), v4.19 and later

[bug] userspace hitting sporadic SIGBUS on xfs (Power9, ppc64le), v4.19 and later

[Jan Stancek]

Hi,

(This bug report is summary from thread [1] with some additions)

User-space binaries on Power9 ppc64le (with 64k pages) on xfs
filesystem are sporadically hitting SIGBUS:

---------- 8< ----------
(gdb) r
Starting program: /mnt/testarea/ltp/testcases/bin/genasin

Program received signal SIGBUS, Bus error.
dl_main (phdr=0x10000040, phnum=<optimized out>, user_entry=0x7fffffffe760, auxv=<optimized out>) at rtld.c:1362
1362        switch (ph->p_type)

(gdb) p ph
$1 = (const Elf64_Phdr *) 0x10000040

(gdb) p *ph
Cannot access memory at address 0x10000040

(gdb) info proc map
process 1110670
Mapped address spaces:

          Start Addr           End Addr       Size     Offset objfile
          0x10000000         0x10010000    0x10000        0x0 /mnt/testarea/ltp/testcases/bin/genasin
          0x10010000         0x10030000    0x20000        0x0 /mnt/testarea/ltp/testcases/bin/genasin
      0x7ffff7f90000     0x7ffff7fb0000    0x20000        0x0 [vdso]
      0x7ffff7fb0000     0x7ffff7fe0000    0x30000        0x0 /usr/lib64/ld-2.30.so
      0x7ffff7fe0000     0x7ffff8000000    0x20000    0x20000 /usr/lib64/ld-2.30.so
      0x7ffffffd0000     0x800000000000    0x30000        0x0 [stack]

(gdb) x/1x 0x10000040
0x10000040:     Cannot access memory at address 0x10000040
---------- >8 ----------

When this happens the binary continues to hit SIGBUS until page
is released, for example by: echo 3 > /proc/sys/vm/drop_caches

The issue goes back to at least v4.19.

I can semi-reliably reproduce it with LTP is installed to /mnt/testarea/ltp by:
while [ True ]; do
        echo 3 > /proc/sys/vm/drop_caches
        rm -f /mnt/testarea/ltp/results/RUNTEST.log /mnt/testarea/ltp/output/RUNTEST.run.log
        ./runltp -p -d results -l RUNTEST.log -o RUNTEST.run.log -f math
        grep FAIL /mnt/testarea/ltp/results/RUNTEST.log && exit 1
done

and some stress activity in other terminal (e.g. kernel build).
Sometimes in minutes, sometimes in hours. It is not reliable
enough to get meaningful bisect results.

My theory is that there's a race in iomap. There appear to be
interleaved calls to iomap_set_range_uptodate() for same page
with varying offset and length. Each call sees bitmap as _not_
entirely "uptodate" and hence doesn't call SetPageUptodate().
Even though each bit in bitmap ends up uptodate by the time
all calls finish.

For example, with following traces:

iomap_set_range_uptodate()
...
        if (uptodate && !PageError(page))
                SetPageUptodate(page);
+       
+       if (mycheck(page)) {
+               trace_printk("page: %px, iop: %px, uptodate: %d, !PageError(page): %d, flags: %lx\n", page, iop, uptodate, !PageError(page), page->flags);
+               trace_printk("first: %u, last: %u, off: %u, len: %u, i: %u\n", first, last, off, len, i);
+       }

I get:
         genacos-18471 [057] ....   162.465730: iomap_readpages: mapping: c000003f185a1ab0
         genacos-18471 [057] ....   162.465732: iomap_page_create: iomap_page_create page: c00c00000fe26180, page->private: 0000000000000000, iop: c000003fc70a19c0, flags: 3ffff800000001
         genacos-18471 [057] ....   162.465736: iomap_set_range_uptodate: page: c00c00000fe26180, iop: c000003fc70a19c0, uptodate: 0, !PageError(page): 1, flags: 3ffff800002001
         genacos-18471 [057] ....   162.465736: iomap_set_range_uptodate: first: 1, last: 14, off: 4096, len: 57344, i: 16
          <idle>-0     [060] ..s.   162.534862: iomap_set_range_uptodate: page: c00c00000fe26180, iop: c000003fc70a19c0, uptodate: 0, !PageError(page): 1, flags: 3ffff800002081
          <idle>-0     [061] ..s.   162.534862: iomap_set_range_uptodate: page: c00c00000fe26180, iop: c000003fc70a19c0, uptodate: 0, !PageError(page): 1, flags: 3ffff800002081
          <idle>-0     [060] ..s.   162.534864: iomap_set_range_uptodate: first: 0, last: 0, off: 0, len: 4096, i: 16
          <idle>-0     [061] ..s.   162.534864: iomap_set_range_uptodate: first: 15, last: 15, off: 61440, len: 4096, i: 16

This page doesn't have Uptodate flag set, which leads to filemap_fault()
returning VM_FAULT_SIGBUS:

crash> p/x ((struct page *) 0xc00c00000fe26180)->flags                                                                                                                                             
$1 = 0x3ffff800002032

crash> kmem -g 0x3ffff800002032
FLAGS: 3ffff800002032
  PAGE-FLAG       BIT  VALUE
  PG_error          1  0000002
  PG_dirty          4  0000010
  PG_lru            5  0000020
  PG_private_2     13  0002000
  PG_fscache       13  0002000
  PG_savepinned     4  0000010
  PG_double_map    13  0002000

But iomap_page->uptodate in page->private suggests all bits are uptodate:

crash> p/x ((struct page *) 0xc00c00000fe26180)->private
$2 = 0xc000003fc70a19c0

crash> p/x ((struct iomap_page *) 0xc000003fc70a19c0)->uptodate                                                                                                                                    
$3 = {0xffff, 0x0}


It appears (after ~4 hours) that I can avoid the problem if I split
the loop so that bits are checked only after all updates are made.
Not sure if this correct approach, or just making it less reproducible:

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index e25901ae3ff4..abe37031c93d 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -131,7 +131,11 @@ iomap_set_range_uptodate(struct page *page, unsigned off, unsigned len)
                for (i = 0; i < PAGE_SIZE / i_blocksize(inode); i++) {
                        if (i >= first && i <= last)
                                set_bit(i, iop->uptodate);
-                       else if (!test_bit(i, iop->uptodate))
+               }
+               for (i = 0; i < PAGE_SIZE / i_blocksize(inode); i++) {
+                       if (i >= first && i <= last)
+                               continue;
+                       if (!test_bit(i, iop->uptodate))
                                uptodate = false;
                }
        }

Thanks,
Jan

[1] https://lore.kernel.org/stable/1420623640.14527843.1575289859701.JavaMail.zimbra@redhat.com/T/#u

Re: [bug] userspace hitting sporadic SIGBUS on xfs (Power9, ppc64le), v4.19 and later

[Christoph Hellwig]

On Tue, Dec 03, 2019 at 07:50:39AM -0500, Jan Stancek wrote:
> My theory is that there's a race in iomap. There appear to be
> interleaved calls to iomap_set_range_uptodate() for same page
> with varying offset and length. Each call sees bitmap as _not_
> entirely "uptodate" and hence doesn't call SetPageUptodate().
> Even though each bit in bitmap ends up uptodate by the time
> all calls finish.

Weird.  That should be prevented by the page lock that all callers
of iomap_set_range_uptodate.  But in case I miss something, does
the patch below trigger?  If not it is not jut a race, but might
be some weird ordering problem with the bitops, especially if it
only triggers on ppc, which is very weakly ordered.

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index d33c7bc5ee92..25e942c71590 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -148,6 +148,8 @@ iomap_set_range_uptodate(struct page *page, unsigned off, unsigned len)
 	unsigned int i;
 	bool uptodate = true;
 
+	WARN_ON_ONCE(!PageLocked(page));
+
 	if (iop) {
 		for (i = 0; i < PAGE_SIZE / i_blocksize(inode); i++) {
 			if (i >= first && i <= last)

Re: [bug] userspace hitting sporadic SIGBUS on xfs (Power9, ppc64le), v4.19 and later

[Jan Stancek]

----- Original Message -----
> On Tue, Dec 03, 2019 at 07:50:39AM -0500, Jan Stancek wrote:
> > My theory is that there's a race in iomap. There appear to be
> > interleaved calls to iomap_set_range_uptodate() for same page
> > with varying offset and length. Each call sees bitmap as _not_
> > entirely "uptodate" and hence doesn't call SetPageUptodate().
> > Even though each bit in bitmap ends up uptodate by the time
> > all calls finish.
> 
> Weird.  That should be prevented by the page lock that all callers
> of iomap_set_range_uptodate.  But in case I miss something, does
> the patch below trigger?  If not it is not jut a race, but might
> be some weird ordering problem with the bitops, especially if it
> only triggers on ppc, which is very weakly ordered.
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index d33c7bc5ee92..25e942c71590 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -148,6 +148,8 @@ iomap_set_range_uptodate(struct page *page, unsigned off,
> unsigned len)
>  	unsigned int i;
>  	bool uptodate = true;
>  
> +	WARN_ON_ONCE(!PageLocked(page));
> +
>  	if (iop) {
>  		for (i = 0; i < PAGE_SIZE / i_blocksize(inode); i++) {
>  			if (i >= first && i <= last)
> 

Hit it pretty quick this time:

# uptime
 09:27:42 up 22 min,  2 users,  load average: 0.09, 13.38, 26.18

# /mnt/testarea/ltp/testcases/bin/genbessel                                                                                                                                     
Bus error (core dumped)

# dmesg | grep -i -e warn -e call                                                                                                                                               
[    0.000000] dt-cpu-ftrs: not enabling: system-call-vectored (disabled or unsupported by kernel)
[    0.000000] random: get_random_u64 called from cache_random_seq_create+0x98/0x1e0 with crng_init=0
[    0.000000] rcu:     Offload RCU callbacks from CPUs: (none).
[    5.312075] megaraid_sas 0031:01:00.0: megasas_disable_intr_fusion is called outbound_intr_mask:0x40000009
[    5.357307] megaraid_sas 0031:01:00.0: megasas_disable_intr_fusion is called outbound_intr_mask:0x40000009
[    5.485126] megaraid_sas 0031:01:00.0: megasas_enable_intr_fusion is called outbound_intr_mask:0x40000000

So, extra WARN_ON_ONCE applied on top of v5.4-8836-g81b6b96475ac
did not trigger.

Is it possible for iomap code to submit multiple bio-s for same
locked page and then receive callbacks in parallel?

Re: [bug] userspace hitting sporadic SIGBUS on xfs (Power9, ppc64le), v4.19 and later

[Darrick J. Wong]

On Tue, Dec 03, 2019 at 09:35:28AM -0500, Jan Stancek wrote:
> 
> ----- Original Message -----
> > On Tue, Dec 03, 2019 at 07:50:39AM -0500, Jan Stancek wrote:
> > > My theory is that there's a race in iomap. There appear to be
> > > interleaved calls to iomap_set_range_uptodate() for same page
> > > with varying offset and length. Each call sees bitmap as _not_
> > > entirely "uptodate" and hence doesn't call SetPageUptodate().
> > > Even though each bit in bitmap ends up uptodate by the time
> > > all calls finish.
> > 
> > Weird.  That should be prevented by the page lock that all callers
> > of iomap_set_range_uptodate.  But in case I miss something, does
> > the patch below trigger?  If not it is not jut a race, but might
> > be some weird ordering problem with the bitops, especially if it
> > only triggers on ppc, which is very weakly ordered.
> > 
> > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > index d33c7bc5ee92..25e942c71590 100644
> > --- a/fs/iomap/buffered-io.c
> > +++ b/fs/iomap/buffered-io.c
> > @@ -148,6 +148,8 @@ iomap_set_range_uptodate(struct page *page, unsigned off,
> > unsigned len)
> >  	unsigned int i;
> >  	bool uptodate = true;
> >  
> > +	WARN_ON_ONCE(!PageLocked(page));
> > +
> >  	if (iop) {
> >  		for (i = 0; i < PAGE_SIZE / i_blocksize(inode); i++) {
> >  			if (i >= first && i <= last)
> > 
> 
> Hit it pretty quick this time:
> 
> # uptime
>  09:27:42 up 22 min,  2 users,  load average: 0.09, 13.38, 26.18
> 
> # /mnt/testarea/ltp/testcases/bin/genbessel                                                                                                                                     
> Bus error (core dumped)
> 
> # dmesg | grep -i -e warn -e call                                                                                                                                               
> [    0.000000] dt-cpu-ftrs: not enabling: system-call-vectored (disabled or unsupported by kernel)
> [    0.000000] random: get_random_u64 called from cache_random_seq_create+0x98/0x1e0 with crng_init=0
> [    0.000000] rcu:     Offload RCU callbacks from CPUs: (none).
> [    5.312075] megaraid_sas 0031:01:00.0: megasas_disable_intr_fusion is called outbound_intr_mask:0x40000009
> [    5.357307] megaraid_sas 0031:01:00.0: megasas_disable_intr_fusion is called outbound_intr_mask:0x40000009
> [    5.485126] megaraid_sas 0031:01:00.0: megasas_enable_intr_fusion is called outbound_intr_mask:0x40000000
> 
> So, extra WARN_ON_ONCE applied on top of v5.4-8836-g81b6b96475ac
> did not trigger.
> 
> Is it possible for iomap code to submit multiple bio-s for same
> locked page and then receive callbacks in parallel?

Yes, if (say) you have 64k pages on a 4k-block filesystem and the extent
mapping for all 16 blocks aren't contiguous, then iomap will issue
separate bios for each physical fragment it finds.  iomap will call
submit_bio on those bios whenever it thinks it's done filling the bio,
so you can indeed get multiple callbacks in parallel.

--D

Re: [bug] userspace hitting sporadic SIGBUS on xfs (Power9, ppc64le), v4.19 and later

[Christoph Hellwig]

Please try the patch below:

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 512856a88106..340c15400423 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -28,6 +28,7 @@
 struct iomap_page {
 	atomic_t		read_count;
 	atomic_t		write_count;
+	spinlock_t		uptodate_lock;
 	DECLARE_BITMAP(uptodate, PAGE_SIZE / 512);
 };
 
@@ -51,6 +52,7 @@ iomap_page_create(struct inode *inode, struct page *page)
 	iop = kmalloc(sizeof(*iop), GFP_NOFS | __GFP_NOFAIL);
 	atomic_set(&iop->read_count, 0);
 	atomic_set(&iop->write_count, 0);
+	spin_lock_init(&iop->uptodate_lock);
 	bitmap_zero(iop->uptodate, PAGE_SIZE / SECTOR_SIZE);
 
 	/*
@@ -139,25 +141,38 @@ iomap_adjust_read_range(struct inode *inode, struct iomap_page *iop,
 }
 
 static void
-iomap_set_range_uptodate(struct page *page, unsigned off, unsigned len)
+iomap_iop_set_range_uptodate(struct page *page, unsigned off, unsigned len)
 {
 	struct iomap_page *iop = to_iomap_page(page);
 	struct inode *inode = page->mapping->host;
 	unsigned first = off >> inode->i_blkbits;
 	unsigned last = (off + len - 1) >> inode->i_blkbits;
-	unsigned int i;
 	bool uptodate = true;
+	unsigned long flags;
+	unsigned int i;
 
-	if (iop) {
-		for (i = 0; i < PAGE_SIZE / i_blocksize(inode); i++) {
-			if (i >= first && i <= last)
-				set_bit(i, iop->uptodate);
-			else if (!test_bit(i, iop->uptodate))
-				uptodate = false;
-		}
+	spin_lock_irqsave(&iop->uptodate_lock, flags);
+	for (i = 0; i < PAGE_SIZE / i_blocksize(inode); i++) {
+		if (i >= first && i <= last)
+			set_bit(i, iop->uptodate);
+		else if (!test_bit(i, iop->uptodate))
+			uptodate = false;
 	}
 
-	if (uptodate && !PageError(page))
+	if (uptodate)
+		SetPageUptodate(page);
+	spin_unlock_irqrestore(&iop->uptodate_lock, flags);
+}
+
+static void
+iomap_set_range_uptodate(struct page *page, unsigned off, unsigned len)
+{
+	if (PageError(page))
+		return;
+
+	if (page_has_private(page))
+		iomap_iop_set_range_uptodate(page, off, len);
+	else
 		SetPageUptodate(page);
 }
 

Re: [bug] userspace hitting sporadic SIGBUS on xfs (Power9, ppc64le), v4.19 and later

[Jan Stancek]

----- Original Message -----
> Please try the patch below:

I ran reproducer for 18 hours on 2 systems were it previously reproduced,
there were no crashes / SIGBUS.

Re: [bug] userspace hitting sporadic SIGBUS on xfs (Power9, ppc64le), v4.19 and later

[Jan Stancek]

----- Original Message -----
> 
> 
> On 12/6/19 6:09 PM, dftxbs3e wrote:
> > Hello!
> > 
> > I am very happy that someone has found this issue.
> > 
> > I have been suffering from rather random SIGBUS errors in similar
> > conditions described by the author.
> > 
> > I don't have much troubleshooting information to provide, however, I hit
> > the issue regularly so I could investigate during that.
> > 
> > How do you debug such an issue? I tried a debugger etc. but besides
> > crashing with SIGBUS, I couldnt get any other meaningful information.

If it's same issue, you could check if dropping caches helps.
Figure out what page is it with crash or systemtap and look at page->flags
and ((struct iomap_page *)page->private)->uptodate bitmap.

> 
> You may want to test the patch Christoph sent on the original thread for
> this issue.

Or v5.5-rc1, Christoph's patch has been merged:
  1cea335d1db1 ("iomap: fix sub-page uptodate handling")

Re: [bug] userspace hitting sporadic SIGBUS on xfs (Power9, ppc64le), v4.19 and later

[Eric Sandeen]

On 12/6/19 6:09 PM, dftxbs3e wrote:
> Hello!
> 
> I am very happy that someone has found this issue.
> 
> I have been suffering from rather random SIGBUS errors in similar
> conditions described by the author.
> 
> I don't have much troubleshooting information to provide, however, I hit
> the issue regularly so I could investigate during that.
> 
> How do you debug such an issue? I tried a debugger etc. but besides
> crashing with SIGBUS, I couldnt get any other meaningful information.

You may want to test the patch Christoph sent on the original thread for
this issue.

-Eric

[bug] userspace hitting sporadic SIGBUS on xfs (Power9, ppc64le), v4.19 and later

[dftxbs3e]

Hello!

I am very happy that someone has found this issue.

I have been suffering from rather random SIGBUS errors in similar
conditions described by the author.

I don't have much troubleshooting information to provide, however, I hit
the issue regularly so I could investigate during that.

How do you debug such an issue? I tried a debugger etc. but besides
crashing with SIGBUS, I couldnt get any other meaningful information.

dftxbs3e

On 12/3/19 1:50 PM, Jan Stancek wrote:
> Hi,
>
> (This bug report is summary from thread [1] with some additions)
>
> User-space binaries on Power9 ppc64le (with 64k pages) on xfs
> filesystem are sporadically hitting SIGBUS:
>
> ---------- 8< ----------
> (gdb) r
> Starting program: /mnt/testarea/ltp/testcases/bin/genasin
>
> Program received signal SIGBUS, Bus error.
> dl_main (phdr=0x10000040, phnum=<optimized out>, user_entry=0x7fffffffe760, auxv=<optimized out>) at rtld.c:1362
> 1362        switch (ph->p_type)
>
> (gdb) p ph
> $1 = (const Elf64_Phdr *) 0x10000040
>
> (gdb) p *ph
> Cannot access memory at address 0x10000040
>
> (gdb) info proc map
> process 1110670
> Mapped address spaces:
>
>             Start Addr           End Addr       Size     Offset objfile
>             0x10000000         0x10010000    0x10000        0x0 /mnt/testarea/ltp/testcases/bin/genasin
>             0x10010000         0x10030000    0x20000        0x0 /mnt/testarea/ltp/testcases/bin/genasin
>         0x7ffff7f90000     0x7ffff7fb0000    0x20000        0x0 [vdso]
>         0x7ffff7fb0000     0x7ffff7fe0000    0x30000        0x0 /usr/lib64/ld-2.30.so
>         0x7ffff7fe0000     0x7ffff8000000    0x20000    0x20000 /usr/lib64/ld-2.30.so
>         0x7ffffffd0000     0x800000000000    0x30000        0x0 [stack]
>
> (gdb) x/1x 0x10000040
> 0x10000040:     Cannot access memory at address 0x10000040
> ---------- >8 ----------
>
> When this happens the binary continues to hit SIGBUS until page
> is released, for example by: echo 3 > /proc/sys/vm/drop_caches
>
> The issue goes back to at least v4.19.
>
> I can semi-reliably reproduce it with LTP is installed to /mnt/testarea/ltp by:
> while [ True ]; do
>           echo 3 > /proc/sys/vm/drop_caches
>           rm -f /mnt/testarea/ltp/results/RUNTEST.log /mnt/testarea/ltp/output/RUNTEST.run.log
>           ./runltp -p -d results -l RUNTEST.log -o RUNTEST.run.log -f math
>           grep FAIL /mnt/testarea/ltp/results/RUNTEST.log && exit 1
> done
>
> and some stress activity in other terminal (e.g. kernel build).
> Sometimes in minutes, sometimes in hours. It is not reliable
> enough to get meaningful bisect results.
>
> My theory is that there's a race in iomap. There appear to be
> interleaved calls to iomap_set_range_uptodate() for same page
> with varying offset and length. Each call sees bitmap as _not_
> entirely "uptodate" and hence doesn't call SetPageUptodate().
> Even though each bit in bitmap ends up uptodate by the time
> all calls finish.
>
> For example, with following traces:
>
> iomap_set_range_uptodate()
> ...
>           if (uptodate && !PageError(page))
>                   SetPageUptodate(page);
> +
> +       if (mycheck(page)) {
> +               trace_printk("page: %px, iop: %px, uptodate: %d, !PageError(page): %d, flags: %lx\n", page, iop, uptodate, !PageError(page), page->flags);
> +               trace_printk("first: %u, last: %u, off: %u, len: %u, i: %u\n", first, last, off, len, i);
> +       }
>
> I get:
>            genacos-18471 [057] ....   162.465730: iomap_readpages: mapping: c000003f185a1ab0
>            genacos-18471 [057] ....   162.465732: iomap_page_create: iomap_page_create page: c00c00000fe26180, page->private: 0000000000000000, iop: c000003fc70a19c0, flags: 3ffff800000001
>            genacos-18471 [057] ....   162.465736: iomap_set_range_uptodate: page: c00c00000fe26180, iop: c000003fc70a19c0, uptodate: 0, !PageError(page): 1, flags: 3ffff800002001
>            genacos-18471 [057] ....   162.465736: iomap_set_range_uptodate: first: 1, last: 14, off: 4096, len: 57344, i: 16
>             <idle>-0     [060] ..s.   162.534862: iomap_set_range_uptodate: page: c00c00000fe26180, iop: c000003fc70a19c0, uptodate: 0, !PageError(page): 1, flags: 3ffff800002081
>             <idle>-0     [061] ..s.   162.534862: iomap_set_range_uptodate: page: c00c00000fe26180, iop: c000003fc70a19c0, uptodate: 0, !PageError(page): 1, flags: 3ffff800002081
>             <idle>-0     [060] ..s.   162.534864: iomap_set_range_uptodate: first: 0, last: 0, off: 0, len: 4096, i: 16
>             <idle>-0     [061] ..s.   162.534864: iomap_set_range_uptodate: first: 15, last: 15, off: 61440, len: 4096, i: 16
>
> This page doesn't have Uptodate flag set, which leads to filemap_fault()
> returning VM_FAULT_SIGBUS:
>
> crash> p/x ((struct page *) 0xc00c00000fe26180)->flags
> $1 = 0x3ffff800002032
>
> crash> kmem -g 0x3ffff800002032
> FLAGS: 3ffff800002032
>     PAGE-FLAG       BIT  VALUE
>     PG_error          1  0000002
>     PG_dirty          4  0000010
>     PG_lru            5  0000020
>     PG_private_2     13  0002000
>     PG_fscache       13  0002000
>     PG_savepinned     4  0000010
>     PG_double_map    13  0002000
>
> But iomap_page->uptodate in page->private suggests all bits are uptodate:
>
> crash> p/x ((struct page *) 0xc00c00000fe26180)->private
> $2 = 0xc000003fc70a19c0
>
> crash> p/x ((struct iomap_page *) 0xc000003fc70a19c0)->uptodate
> $3 = {0xffff, 0x0}
>
>
> It appears (after ~4 hours) that I can avoid the problem if I split
> the loop so that bits are checked only after all updates are made.
> Not sure if this correct approach, or just making it less reproducible:
>
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index e25901ae3ff4..abe37031c93d 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -131,7 +131,11 @@ iomap_set_range_uptodate(struct page *page, unsigned off, unsigned len)
>                   for (i = 0; i < PAGE_SIZE / i_blocksize(inode); i++) {
>                           if (i >= first && i <= last)
>                                   set_bit(i, iop->uptodate);
> -                       else if (!test_bit(i, iop->uptodate))
> +               }
> +               for (i = 0; i < PAGE_SIZE / i_blocksize(inode); i++) {
> +                       if (i >= first && i <= last)
> +                               continue;
> +                       if (!test_bit(i, iop->uptodate))
>                                   uptodate = false;
>                   }
>           }
>
> Thanks,
> Jan
>
> [1] https://lore.kernel.org/stable/1420623640.14527843.1575289859701.JavaMail.zimbra@redhat.com/T/#u
>
>