Re: mm/swap: possible problem introduced when replacing REQ_NOIDLE with REQ_IDLE

[Pavel Tikhomirov <ptikhomirov@virtuozzo.com>]

Christoph, a2b809672ee6 is your patch, maybe you have some ideas about 
these problem?

ping

On 8/9/19 11:39 AM, Pavel Tikhomirov wrote:
> Hi, all.
> 
> Then porting patches from mainstream I've found some strange code:
> 
>   > commit a2b809672ee6fcb4d5756ea815725b3dbaea654e
>   > Author: Christoph Hellwig <hch@lst.de>
>   > Date:   Tue Nov 1 07:40:09 2016 -0600
>   >
>   >     block: replace REQ_NOIDLE with REQ_IDLE
>   >
>   >     Noidle should be the default for writes as seen by all the compounds
>   >     definitions in fs.h using it.  In fact only direct I/O really should
>   >     be using NODILE, so turn the whole flag around to get the defaults
>   >     right, which will make our life much easier especially onces the
>   >     WRITE_* defines go away.
>   >
>   >     This assumes all the existing "raw" users of REQ_SYNC for writes
>   >     want noidle behavior, which seems to be spot on from a quick audit.
>   >
>   >     Signed-off-by: Christoph Hellwig <hch@lst.de>
>   >     Signed-off-by: Jens Axboe <axboe@fb.com>
>   >
>   > diff --git a/include/linux/fs.h b/include/linux/fs.h
>   > index ccedccb28ec8..46a74209917f 100644
>   > --- a/include/linux/fs.h
>   > +++ b/include/linux/fs.h
>   > @@ -197,11 +197,11 @@ typedef int (dio_iodone_t)(struct kiocb *iocb,
> loff_t offset,
>   >  #define WRITE                  REQ_OP_WRITE
>   >
>   >  #define READ_SYNC              0
>   > -#define WRITE_SYNC             (REQ_SYNC | REQ_NOIDLE)
>   > -#define WRITE_ODIRECT          REQ_SYNC
>   > -#define WRITE_FLUSH            (REQ_NOIDLE | REQ_PREFLUSH)
>   > -#define WRITE_FUA              (REQ_NOIDLE | REQ_FUA)
>   > -#define WRITE_FLUSH_FUA                (REQ_NOIDLE | REQ_PREFLUSH |
> REQ_FUA)
>   > +#define WRITE_SYNC             REQ_SYNC
>   > +#define WRITE_ODIRECT          (REQ_SYNC | REQ_IDLE)
>   > +#define WRITE_FLUSH            REQ_PREFLUSH
>   > +#define WRITE_FUA              REQ_FUA
>   > +#define WRITE_FLUSH_FUA                (REQ_PREFLUSH | REQ_FUA)
>   >
>   >  /*
>   >   * Attribute flags.  These should be or-ed together to figure out what
> 
> The above commit changes the meaning of the REQ_SYNC flag, before the
> patch it was equal to WRITE_ODIRECT and after the patch it is equal to
> WRITE_SYNC. And thus I think it became treated differently (I see only
> one place left in wbt_should_throttle.).
> 
> But in __swap_writepage() both before and after the mentioned patch we
> still pass a single REQ_SYNC without any REQ_IDLE/REQ_UNIDLE:
> 
>   > [snorch@snorch linux]$ git blame
> a2b809672ee6fcb4d5756ea815725b3dbaea654e^ mm/page_io.c | grep -a5 REQ_SYNC
>   > ^1da177e4c3f4 (Linus Torvalds        2005-04-16 15:20:36 -0700 319)
>           unlock_page(page);
>   > ^1da177e4c3f4 (Linus Torvalds        2005-04-16 15:20:36 -0700 320)
>           ret = -ENOMEM;
>   > ^1da177e4c3f4 (Linus Torvalds        2005-04-16 15:20:36 -0700 321)
>           goto out;
>   > ^1da177e4c3f4 (Linus Torvalds        2005-04-16 15:20:36 -0700 322)   }
>   > ^1da177e4c3f4 (Linus Torvalds        2005-04-16 15:20:36 -0700 323)
> if (wbc->sync_mode == WB_SYNC_ALL)
>   > ba13e83ec334c (Jens Axboe            2016-08-01 09:38:44 -0600 324)
>           bio_set_op_attrs(bio, REQ_OP_WRITE, REQ_SYNC);
>   > ba13e83ec334c (Jens Axboe            2016-08-01 09:38:44 -0600 325)
> else
>   > ba13e83ec334c (Jens Axboe            2016-08-01 09:38:44 -0600 326)
>           bio_set_op_attrs(bio, REQ_OP_WRITE, 0);
>   > f8891e5e1f93a (Christoph Lameter     2006-06-30 01:55:45 -0700 327)
> count_vm_event(PSWPOUT);
>   > ^1da177e4c3f4 (Linus Torvalds        2005-04-16 15:20:36 -0700 328)
> set_page_writeback(page);
>   > ^1da177e4c3f4 (Linus Torvalds        2005-04-16 15:20:36 -0700 329)
> unlock_page(page);
>   > [snorch@snorch linux]$ git blame
> a2b809672ee6fcb4d5756ea815725b3dbaea654e mm/page_io.c | grep -a5 REQ_SYNC
>   > ^1da177e4c3f4 (Linus Torvalds        2005-04-16 15:20:36 -0700 319)
>           unlock_page(page);
>   > ^1da177e4c3f4 (Linus Torvalds        2005-04-16 15:20:36 -0700 320)
>           ret = -ENOMEM;
>   > ^1da177e4c3f4 (Linus Torvalds        2005-04-16 15:20:36 -0700 321)
>           goto out;
>   > ^1da177e4c3f4 (Linus Torvalds        2005-04-16 15:20:36 -0700 322)   }
>   > ^1da177e4c3f4 (Linus Torvalds        2005-04-16 15:20:36 -0700 323)
> if (wbc->sync_mode == WB_SYNC_ALL)
>   > ba13e83ec334c (Jens Axboe            2016-08-01 09:38:44 -0600 324)
>           bio_set_op_attrs(bio, REQ_OP_WRITE, REQ_SYNC);
>   > ba13e83ec334c (Jens Axboe            2016-08-01 09:38:44 -0600 325)
> else
>   > ba13e83ec334c (Jens Axboe            2016-08-01 09:38:44 -0600 326)
>           bio_set_op_attrs(bio, REQ_OP_WRITE, 0);
>   > f8891e5e1f93a (Christoph Lameter     2006-06-30 01:55:45 -0700 327)
> count_vm_event(PSWPOUT);
>   > ^1da177e4c3f4 (Linus Torvalds        2005-04-16 15:20:36 -0700 328)
> set_page_writeback(page);
>   > ^1da177e4c3f4 (Linus Torvalds        2005-04-16 15:20:36 -0700 329)
> unlock_page(page);
> 
> It looks like we've changed the way how we handle swap page writes from
> "odirect" way to "regular" sync write way, these can be wrong. This may
> also affect deprecated cfq io-scheduler on older kernels.
> 
> Thanks in advance for any advice on what to do with these, may be I miss
> something.
> 

-- 
Best regards, Tikhomirov Pavel
Software Developer, Virtuozzo.