Re: [PATCH 2/2] f2fs: support data compression

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Oct 30, 2019 at 04:43:52PM +0800, Chao Yu wrote:
> On 2019/10/30 10:55, Eric Biggers wrote:
> > On Tue, Oct 29, 2019 at 04:33:36PM +0800, Chao Yu wrote:
> >> On 2019/10/28 6:50, Eric Biggers wrote:
> >>>> +bool f2fs_is_compressed_page(struct page *page)
> >>>> +{
> >>>> +	if (!page_private(page))
> >>>> +		return false;
> >>>> +	if (IS_ATOMIC_WRITTEN_PAGE(page) || IS_DUMMY_WRITTEN_PAGE(page))
> >>>> +		return false;
> >>>> +	return *((u32 *)page_private(page)) == F2FS_COMPRESSED_PAGE_MAGIC;
> >>>> +}
> >>>
> >>> This code implies that there can be multiple page private structures each of
> >>> which has a different magic number.  But I only see F2FS_COMPRESSED_PAGE_MAGIC.
> >>> Where in the code is the other one(s)?
> >>
> >> I'm not sure I understood you correctly, did you mean it needs to introduce
> >> f2fs_is_atomic_written_page() and f2fs_is_dummy_written_page() like
> >> f2fs_is_compressed_page()?
> >>
> > 
> > No, I'm asking what is the case where the line
> > 
> > 	*((u32 *)page_private(page)) == F2FS_COMPRESSED_PAGE_MAGIC
> > 
> > returns false?
> 
> Should be this?
> 
> if (!page_private(page))
> 	return false;
> f2fs_bug_on(*((u32 *)page_private(page)) != F2FS_COMPRESSED_PAGE_MAGIC)
> return true;

Yes, that makes more sense, unless there are other cases.

> 
> > 
> >>>
> >>>> +
> >>>> +static void f2fs_set_compressed_page(struct page *page,
> >>>> +		struct inode *inode, pgoff_t index, void *data, refcount_t *r)
> >>>> +{
> >>>> +	SetPagePrivate(page);
> >>>> +	set_page_private(page, (unsigned long)data);
> >>>> +
> >>>> +	/* i_crypto_info and iv index */
> >>>> +	page->index = index;
> >>>> +	page->mapping = inode->i_mapping;
> >>>> +	if (r)
> >>>> +		refcount_inc(r);
> >>>> +}
> >>>
> >>> It isn't really appropriate to create fake pagecache pages like this.  Did you
> >>> consider changing f2fs to use fscrypt_decrypt_block_inplace() instead?
> >>
> >> We need to store i_crypto_info and iv index somewhere, in order to pass them to
> >> fscrypt_decrypt_block_inplace(), where did you suggest to store them?
> >>
> > 
> > The same place where the pages are stored.
> 
> Still we need allocate space for those fields, any strong reason to do so?
> 

page->mapping set implies that the page is a pagecache page.  Faking it could
cause problems with code elsewhere.

> > 
> >>>> +
> >>>> +void f2fs_destroy_compress_ctx(struct compress_ctx *cc)
> >>>> +{
> >>>> +	kvfree(cc->rpages);
> >>>> +}
> >>>
> >>> The memory is allocated with kzalloc(), so why is it freed with kvfree() and not
> >>> just kfree()?
> >>
> >> It was allocated by f2fs_*alloc() which will fallback to kvmalloc() once
> >> kmalloc() failed.
> > 
> > This seems to be a bug in f2fs_kmalloc() -- it inappropriately falls back to
> > kvmalloc().  As per its name, it should only use kmalloc().  f2fs_kvmalloc()
> > already exists, so it can be used when the fallback is wanted.
> 
> We can introduce f2fs_memalloc() to wrap f2fs_kmalloc() and f2fs_kvmalloc() as
> below:
> 
> f2fs_memalloc()
> {
> 	mem = f2fs_kmalloc();
> 	if (mem)
> 		return mem;
> 	return f2fs_kvmalloc();
> }
> 
> It can be used in specified place where we really need it, like the place
> descirbied in 5222595d093e ("f2fs: use kvmalloc, if kmalloc is failed") in where
> we introduced original logic.

No, just use kvmalloc().  The whole point of kvmalloc() is that it tries
kmalloc() and then falls back to vmalloc() if it fails.

- Eric