Re: [f2fs-dev] [PATCH v3] f2fs: fix race of pending_pages in decompression

[Eric Biggers]

On Tue, Dec 08, 2020 at 08:51:45AM +0900, Daeho Jeong wrote:
> > I am trying to review this but it is very hard, as the f2fs compression code is
> > very hard to understand.
> >
> > It looks like a 'struct decompress_io_ctx' represents the work to decompress a
> > particular cluster.  Since the compressed data of the cluster can be read using
> > multiple bios, there is a reference count of how many pages are remaining to be
> > read before all the cluster's pages have been read and decompression can start.
> >
> > What I don't understand is why that reference counting needs to work differently
> > depending on whether verity is enabled or not.  Shouldn't it be exactly the
> > same?
> >
> > There also seems to be some confusion about the scope of STEP_VERITY.  Before
> > f2fs compression was added, it was a per-bio thing.  But now in a compressed
> > file, it's really a per-cluster thing, since all decompressed pages in a
> > compressed cluster are verified (or not verified) at once.
> >
> > Wouldn't it make a lot more sense to, when a cluster needs both compression and
> > verity, *not* set STEP_VERITY on the bios, but rather set a similar flag in the
> > decompress_io_ctx?
> >
> 
> Eric,
> 
> Decompression and verity can be executed in different thread contexts
> in different timing, so we need separate counts for each.
> 
> We already use STEP_VERITY for non-compression case, so I think using
> this flag in here looks more making sense.
> 
> Thanks,

That didn't really answer my questions.

I gave up trying to review this patch as the compression post-read handling is
just way too weird and hard to understand.  I wrote a patch to clean it all up
instead, please take a look:
https://lkml.kernel.org/r/20201208060328.2237091-1-ebiggers@xxxxxxxxxx

- Eric