Re: [f2fs-dev] [PATCH] f2fs: don't return vmalloc() memory from f2fs_kmalloc()

[Jaegeuk Kim <jaegeuk@kernel.org>]

On 06/04, Eric Biggers wrote:
> On Mon, May 18, 2020 at 12:10:16PM -0700, Jaegeuk Kim wrote:
> > On 05/18, Eric Biggers wrote:
> > > On Mon, May 18, 2020 at 11:06:48AM -0700, Jaegeuk Kim wrote:
> > > > On 05/05, Eric Biggers wrote:
> > > > > From: Eric Biggers <ebiggers@google.com>
> > > > > 
> > > > > kmalloc() returns kmalloc'ed memory, and kvmalloc() returns either
> > > > > kmalloc'ed or vmalloc'ed memory.  But the f2fs wrappers, f2fs_kmalloc()
> > > > > and f2fs_kvmalloc(), both return both kinds of memory.
> > > > > 
> > > > > It's redundant to have two functions that do the same thing, and also
> > > > > breaking the standard naming convention is causing bugs since people
> > > > > assume it's safe to kfree() memory allocated by f2fs_kmalloc().  See
> > > > > e.g. the various allocations in fs/f2fs/compress.c.
> > > > > 
> > > > > Fix this by making f2fs_kmalloc() just use kmalloc().  And to avoid
> > > > > re-introducing the allocation failures that the vmalloc fallback was
> > > > > intended to fix, convert the largest allocations to use f2fs_kvmalloc().
> > > > > 
> > > > > Signed-off-by: Eric Biggers <ebiggers@google.com>
> > > > > ---
> > > > >  fs/f2fs/checkpoint.c | 4 ++--
> > > > >  fs/f2fs/f2fs.h       | 8 +-------
> > > > >  fs/f2fs/node.c       | 8 ++++----
> > > > >  3 files changed, 7 insertions(+), 13 deletions(-)
> > > > > 
> > > > > diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
> > > > > index 97b6378554b406..ac5b47f15f5e77 100644
> > > > > --- a/fs/f2fs/checkpoint.c
> > > > > +++ b/fs/f2fs/checkpoint.c
> > > > > @@ -895,8 +895,8 @@ int f2fs_get_valid_checkpoint(struct f2fs_sb_info *sbi)
> > > > >  	int i;
> > > > >  	int err;
> > > > >  
> > > > > -	sbi->ckpt = f2fs_kzalloc(sbi, array_size(blk_size, cp_blks),
> > > > > -				 GFP_KERNEL);
> > > > > +	sbi->ckpt = f2fs_kvzalloc(sbi, array_size(blk_size, cp_blks),
> > > > > +				  GFP_KERNEL);
> > > > >  	if (!sbi->ckpt)
> > > > >  		return -ENOMEM;
> > > > >  	/*
> > > > > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> > > > > index d036a31a97e84e..bc4c5b9b1bf14c 100644
> > > > > --- a/fs/f2fs/f2fs.h
> > > > > +++ b/fs/f2fs/f2fs.h
> > > > > @@ -2953,18 +2953,12 @@ static inline bool f2fs_may_extent_tree(struct inode *inode)
> > > > >  static inline void *f2fs_kmalloc(struct f2fs_sb_info *sbi,
> > > > >  					size_t size, gfp_t flags)
> > > > >  {
> > > > > -	void *ret;
> > > > > -
> > > > >  	if (time_to_inject(sbi, FAULT_KMALLOC)) {
> > > > >  		f2fs_show_injection_info(sbi, FAULT_KMALLOC);
> > > > >  		return NULL;
> > > > >  	}
> > > > >  
> > > > > -	ret = kmalloc(size, flags);
> > > > > -	if (ret)
> > > > > -		return ret;
> > > > > -
> > > > > -	return kvmalloc(size, flags);
> > > > > +	return kmalloc(size, flags);
> > > > 
> > > > IIRC, sometimes, we suffered from ENOMEM from kmalloc, as some structures
> > > > depended on the disk capacity. I can't remember exactly which structure tho.
> > > > 
> > > 
> > > I think this patch already addresses that, by changing the large allocations to
> > > use f2fs_kvmalloc().
> > 
> > Hmm, I worried a bit whether it covers every cases.
> > 
> 
> I went through every remaining caller of f2fs_kmalloc() and f2fs_kzalloc().  I
> think we're fine, except for possibly the allocation of blkz_seq in
> init_blkz_info().  How many zones can we expect on a zoned block device?

In the worst case, it can be 16TB / 2MB = 8M entries.

> 
> Other than that, the largest fixed-size allocation is 8536 bytes
> (struct discard_cmd_control).  And the variable-size allocations are all a page
> or less, except for xattr buffers which maybe can be larger, but the VFS uses
> kmalloc() for those too.
> 
> Anyway, f2fs used to allocate megabytes with kmalloc(), so I'm not surprised you
> had issues before.  But that's not a good reason to make *every* caller
> potentially get vmalloc()'ed memory, in the process introducing bugs where
> vmalloc() memory isn't handled correctly.

Thank you for checking all the cases. Let me know, if this is the final version.
I may be able to merge and try to see if somebody will complain later. :P

Thanks,

> 
> - Eric


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