Re: [PATCH] PM: hibernate: Do not get block device exclusively in test_resume mode

[Chen Yu]

Hi Pavan,
On 2023-04-05 at 12:30:00 +0530, Pavan Kondeti wrote:
> On Sun, Apr 02, 2023 at 12:55:40AM +0800, Chen Yu wrote:
> > The system refused to do a test_resume because it found that the
> > swap device has already been taken by someone else. Specificly,
> > the swsusp_check()->blkdev_get_by_dev(FMODE_EXCL) is supposed to
> > do this check.
> > 
> > Steps to reproduce:
> >  dd if=/dev/zero of=/swapfile bs=$(cat /proc/meminfo | 
> >        awk '/MemTotal/ {print $2}') count=1024 conv=notrunc
> >  mkswap /swapfile
> >  swapon /swapfile
> >  swap-offset /swapfile
> >  echo 34816 > /sys/power/resume_offset
> >  echo test_resume > /sys/power/disk
> >  echo disk > /sys/power/state
> > 
> >  PM: Using 3 thread(s) for compression
> >  PM: Compressing and saving image data (293150 pages)...
> >  PM: Image saving progress:   0%
> >  PM: Image saving progress:  10%
> >  ata1: SATA link up 1.5 Gbps (SStatus 113 SControl 300)
> >  ata1.00: configured for UDMA/100
> >  ata2: SATA link down (SStatus 0 SControl 300)
> >  ata5: SATA link down (SStatus 0 SControl 300)
> >  ata6: SATA link down (SStatus 0 SControl 300)
> >  ata3: SATA link down (SStatus 0 SControl 300)
> >  ata4: SATA link down (SStatus 0 SControl 300)
> >  PM: Image saving progress:  20%
> >  PM: Image saving progress:  30%
> >  PM: Image saving progress:  40%
> >  PM: Image saving progress:  50%
> >  pcieport 0000:00:02.5: pciehp: Slot(0-5): No device found
> >  PM: Image saving progress:  60%
> >  PM: Image saving progress:  70%
> >  PM: Image saving progress:  80%
> >  PM: Image saving progress:  90%
> >  PM: Image saving done
> >  PM: hibernation: Wrote 1172600 kbytes in 2.70 seconds (434.29 MB/s)
> >  PM: S|
> >  PM: hibernation: Basic memory bitmaps freed
> >  PM: Image not found (code -16)
> > 
> > This is because when using the swapfile as the hibernation storage,
> > the block device where the swapfile is located has already been mounted
> > by the OS distribution(usually been mounted as the rootfs). This is not
> > an issue for normal hibernation, because software_resume()->swsusp_check()
> > happens before the block device(rootfs) mount. But it is a problem for the
> > test_resume mode. Because when test_resume happens, the block device has
> > been mounted already.
> > 
> > Thus remove the FMODE_EXCL for test_resume mode. This would not be a
> > problem because in test_resume stage, the processes have already been
> > frozen, and the race condition described in
> > Commit 39fbef4b0f77 ("PM: hibernate: Get block device exclusively in swsusp_check()")
> > is unlikely to happen.
> > 
> > Fixes: 39fbef4b0f77 ("PM: hibernate: Get block device exclusively in swsusp_check()")
> > Reported-by: Yifan Li <yifan2.li@xxxxxxxxx>
> > Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> > +int swsusp_check(bool safe)
> >  {
> > +	fmode_t mode = FMODE_READ;
> >  	int error;
> >  	void *holder;
> >  
> > +	if (!safe)
> > +		mode |= FMODE_EXCL;
> > +
> >  	hib_resume_bdev = blkdev_get_by_dev(swsusp_resume_device,
> > -					    FMODE_READ | FMODE_EXCL, &holder);
> > +					    mode, &holder);
> >  	if (!IS_ERR(hib_resume_bdev)) {
> >  		set_blocksize(hib_resume_bdev, PAGE_SIZE);
> >  		clear_page(swsusp_header);
> > @@ -1547,7 +1551,7 @@ int swsusp_check(void)
> >  
> >  put:
> >  		if (error)
> > -			blkdev_put(hib_resume_bdev, FMODE_READ | FMODE_EXCL);
> > +			blkdev_put(hib_resume_bdev, mode);
> >  		else
> >  			pr_debug("Image signature found, resuming\n");
> >  	} else {
> 
> The patch looks good to me and it works. I have just one
> question/comment.
> 
> What is "safe" here? Because I worked on this problem [1], so I
> understood it. but it is not very clear / explicit. 
I see.
> One approach I thought would be to the codepaths aware of "test_resume" via a
> global variable called "snapshot_testing" similar to freezer_test_done.
> if snapshot_testing is true, don't use exclusive flags.
This looks reasonable, with this change, we don't have to add "safe" parameter to
swsusp_check() and load_image_and_restore().

thanks,
Chenyu
> 
> Thanks,
> Pavan
>