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Date: Thu, 13 May 2021 16:27:23 -0400
From: Leah Rumancik <leah.rumancik@...il.com>
To: Theodore Ts'o <tytso@....edu>
Cc: linux-ext4@...r.kernel.org
Subject: Re: [PATCH v4 1/3] ext4: add discard/zeroout flags to journal flush
On Thu, May 13, 2021 at 02:09:26PM -0400, Theodore Ts'o wrote:
> On Tue, May 11, 2021 at 06:04:26PM +0000, Leah Rumancik wrote:
> > @@ -3223,7 +3223,7 @@ static sector_t ext4_bmap(struct address_space *mapping, sector_t block)
> > ext4_clear_inode_state(inode, EXT4_STATE_JDATA);
> > journal = EXT4_JOURNAL(inode);
> > jbd2_journal_lock_updates(journal);
> > - err = jbd2_journal_flush(journal);
> > + err = jbd2_journal_flush(journal, 0);
>
> In the ocfs2 changes, I noticed you are using "false", instead of 0,
> in the second argument to jbd2_journal_flush.
>
> When I looked more closely, the function signature of
> jbd2_journal_flush is also using an unsigned long long for flags,
> which struck me as strange:
>
> > +extern int jbd2_journal_flush(journal_t *journal, unsigned long long flags);
>
> I then noticed that later in the patch series, the ioctl argument is
> taking an unsigned long long and we're passing that straight through
> to jbd2_journal_flush().
>
> First of all, unsigned long long is not very efficient on many
> platforms (especially 32-bit platforms), but also on platforms where
> int is 32 bits. If we don't expect us to need more than 32 flag bits,
> I'd suggest explicit ly using __u32 in ioctl interface. (__u32 is
> fine; it's the use of the base int type which can get us into trouble,
> since int can be either 32 or 64 bits depending on the architecture).
>
Just to make sure I understand correctly, the explicit __u32 is critical
due to the size being read in by the ioctl, specifically through
copy_from_user? When do you switch from __u32 to unsigned long? I don't
see the __* types being carried throughout.
(Also, just got Darrick's reply about the 32 vs. 64. Yes, originally
went with 64 because there was an argument for it. I believe the 32
is likely sufficient, but I don't feel that strongly about this matter)
> Secondly, I'd suggest using a different set of flags for
> jbd2_journal_flush(), which is an internal kernel interface, and the
> EXT4_IOC_CHECKPOINT interface. We might in the future want to add
> some internal flags to jbd2_journal_flush that we do *not* want to
> expose via EXT4_IOC_CHECKPOINT, and so it's best that we keep those
> two interfaces separate.
>
> > diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> > index 2dc944442802..f86929dbca3c 100644
> > --- a/fs/jbd2/journal.c
> > +++ b/fs/jbd2/journal.c
> > @@ -1686,6 +1686,106 @@ static void jbd2_mark_journal_empty(journal_t *journal, int write_op)
> > write_unlock(&journal->j_state_lock);
> > }
> >
> > +#define JBD2_ERASE_FLAG_DISCARD 1
> > +#define JBD2_ERASE_FLAG_ZEROOUT 2
>
> I'd suggest defining these in include/linux/jbd2.h, and giving them
> names like: JBD2_JOURNAL_FLUSH_DISCARD and JBD2_JOURNAL_FLUSH_ERASE...
> (and making the flags parameter an unsigned int).
>
> > + /* flags must be set to either discard or zeroout */
> > + if ((flags & JBD2_ERASE_FLAG_DISCARD & JBD2_ERASE_FLAG_ZEROOUT) || !flags)
> > + return -EINVAL;
>
> The expression (flags & JBD2_ERASE_FLAG_DISCARD & JBD2_ERASE_FLAG_ZEROOUT)
> is always going to evaluate to zero, since (1 & 2) is 0.
>
> What you probably want is something like:
>
> #define JBD2_JOURNAL_FLUSH_DISCARD 0x0001
> #define JBD2_JOURNAL_FLUSH_ZEROOUT 0x0002
> #define JBD2_JOURNAL_FLUSH_VALID 0x0003
>
Why call them JBD2_JOURNAL_FLUSH* instead of JBD2_JOURNAL_ERASE* since
they get passed directly through to the erase function? I feel like it
would be weird if someone wanted to use the erase function directly but
had to use JBD2_JOURNAL_FLUSH* flags.
> if ((flags & ~JBD2_JOURNAL_FLUSH_VALID) ||
> ((flags & JBD2_JOURNAL_FLUSH_DISCARD) &&
> (flags & JBD2_JOURNAL_FLUSH_ZEROOUT)))
> return -EINVAL;
>
Ah, great. Thanks!
> > +
> > + err = jbd2_journal_bmap(journal, log_offset, &block_start);
> > + if (err) {
> > + printk(KERN_ERR "JBD2: bad block at offset %lu", log_offset);
> > + return err;
> > + }
>
> We could get rid of this, and instead make sure block_start is initialized
> to ~((unsigned long long) 0). Then in the loop we can do...
>
> > +
> > + /*
> > + * use block_start - 1 to meet check for contiguous with previous region:
> > + * phys_block == block_stop + 1
> > + */
> > + block_stop = block_start - 1;
> > +
> > + for (block = log_offset; block < journal->j_total_len; block++) {
> > + err = jbd2_journal_bmap(journal, block, &phys_block);
> > + if (err) {
> > + printk(KERN_ERR "JBD2: bad block at offset %lu", block);
> > + return err;
> > + }
>
> if (block_start == ~((unsigned long long) 0)) {
> block_start = phys_block;
> block_Stop = block_start - 1;
> }
>
> > +
> > + if (block == journal->j_total_len - 1) {
> > + block_stop = phys_block;
> > + } else if (phys_block == block_stop + 1) {
> > + block_stop++;
> > + continue;
> > + }
> > +
> > + /*
> > + * not contiguous with prior physical block or this is last
> > + * block of journal, take care of the region
> > + */
> > + byte_start = block_start * journal->j_blocksize;
> > + byte_stop = block_stop * journal->j_blocksize;
> > + byte_count = (block_stop - block_start + 1) *
> > + journal->j_blocksize;
> > +
> > + truncate_inode_pages_range(journal->j_dev->bd_inode->i_mapping,
> > + byte_start, byte_stop);
> > +
> > + if (flags & JBD2_ERASE_FLAG_DISCARD) {
> > + err = blkdev_issue_discard(journal->j_dev,
> > + byte_start >> SECTOR_SHIFT,
> > + byte_count >> SECTOR_SHIFT,
> > + GFP_NOFS, 0);
> > + } else if (flags & JBD2_ERASE_FLAG_ZEROOUT) {
> > + err = blkdev_issue_zeroout(journal->j_dev,
> > + byte_start >> SECTOR_SHIFT,
> > + byte_count >> SECTOR_SHIFT,
> > + GFP_NOFS, 0);
> > + }
> > +
> > + if (unlikely(err != 0)) {
> > + printk(KERN_ERR "JBD2: (error %d) unable to wipe journal at physical blocks %llu - %llu",
> > + err, block_start, block_stop);
> > + return err;
> > + }
> > +
> > + block_start = phys_block;
> > + block_stop = phys_block;
>
> Is this right? When we initialized the loop, above, block_stop was
> set to block_start-1 (where block_start == phys_block). So I think it
> might be more correct to replace the above two lines with:
>
> block_start = ~((unsigned long long) 0);
>
I'll play around with this and see if I can get it to work. Seems like
it might simplify the code a bit.
> ... and then let block_start and block_stop be initialized in a single
> place. Do you agree? Does this make sense to you?
>
> - Ted
Thanks!
Leah
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