From: Gao Xiang <firstname.lastname@example.org>
To: Dave Chinner <email@example.com>,
Goldwyn Rodrigues <RGoldwyn@suse.com>, "firstname.lastname@example.org" <email@example.com>,
Subject: Re: [PATCH 10/13] iomap: use a function pointer for dio submits
Date: Thu, 8 Aug 2019 14:28:26 +0800 [thread overview]
Message-ID: <20190808062825.GC28630@138> (raw)
On Wed, Aug 07, 2019 at 10:49:36PM -0700, Eric Biggers wrote:
> On Thu, Aug 08, 2019 at 12:26:42PM +0800, Gao Xiang wrote:
> > >
> > > > > That's why I don't like this hook - I think hiding data operations
> > > > > and/or custom bio manipulations in opaque filesystem callouts is
> > > > > completely the wrong approach to be taking. We need to do these
> > > > > things in a generic manner so that all filesystems (and block
> > > > > devices!) that use the iomap infrastructure can take advantage of
> > > > > them, not just one of them.
> > > > >
> > > > > Quite frankly, I don't care if it takes more time and work up front,
> > > > > I'm tired of expedient hacks to merge code quickly repeatedly biting
> > > > > us on the arse and wasting far more time sorting out than we would
> > > > > have spent getting it right in the first place.
> > > >
> > > > Sure. I am open to ideas. What are you proposing?
> > >
> > > That you think about how to normalise the btrfs IO path to fit into
> > > the standard iomap/blockdev model, rather than adding special hacks
> > > to iomap to allow an opaque, custom, IO model to be shoe-horned into
> > > the generic code.
> > >
> > > For example, post-read validation requires end-io processing,
> > > whether it be encryption, decompression, CRC/T10 validation, etc. The
> > > iomap end-io completion has all the information needed to run these
> > > things, whether it be a callout to the filesystem for custom
> > > processing checking, or a generic "decrypt into supplied data page"
> > > sort of thing. These all need to be done in the same place, so we
> > > should have common support for this. And I suspect the iomap should
> > > also state in a flag that something like this is necessary (e.g.
> > > IOMAP_FL_ENCRYPTED indicates post-IO decryption needs to be run).
> > Add some word to this topic, I think introducing a generic full approach
> > to IOMAP for encryption, decompression, verification is hard to meet all
> > filesystems, and seems unnecessary, especially data compression is involved.
> > Since the data decompression will expand the data, therefore the logical
> > data size is not same as the physical data size:
> > 1) IO submission should be applied to all physical data, but data
> > decompression will be eventually applied to logical mapping.
> > As for EROFS, it submits all physical pages with page->private
> > points to management structure which maintain all logical pages
> > as well for further decompression. And time-sharing approach is
> > used to save the L2P mapping array in these allocated pages itself.
> > In addition, IOMAP also needs to consider fixed-sized output/input
> > difference which is filesystem specific and I have no idea whether
> > involveing too many code for each requirement is really good for IOMAP;
> > 2) The post-read processing order is another negotiable stuff.
> > Although there is no benefit to select verity->decrypt rather than
> > decrypt->verity; but when compression is involved, the different
> > orders could be selected by different filesystem users:
> > 1. decrypt->verity->decompress
> > 2. verity->decompress->decrypt
> > 3. decompress->decrypt->verity
> > 1. and 2. could cause less computation since it processes
> > compressed data, and the security is good enough since
> > the behavior of decompression algorithm is deterministic.
> > 3 could cause more computation.
> > All I want to say is the post process is so complicated since we have
> > many selection if encryption, decompression, verification are all involved.
> > Maybe introduce a core subset to IOMAP is better for long-term
> > maintainment and better performance. And we should consider it
> > more carefully.
> FWIW, the only order that actually makes sense is decrypt->decompress->verity.
I am not just talking about fsverity as you mentioned below.
> Decrypt before decompress, i.e. encrypt after compress, because only the
> plaintext can be compressible; the ciphertext isn't.
There could be some potential users need partially decrypt/decompress,
but that is minor. I don't want to talk about this detail in this topic.
> Verity last, on the original data, because otherwise the file hash that
> fs-verity reports would be specific to that particular inode on-disk and
> therefore would be useless for authenticating the file's user-visible contents.
> [By "verity" I mean specifically fs-verity. Integrity-only block checksums are
> a different case; those can be done at any point, but doing them on the
> compressed data would make sense as then there would be less to checksum.
> And yes, compression+encryption leaks information about the original data, so
> may not be advisable. My point is just that if the two are nevertheless
> combined, it only makes sense to compress the plaintext.]
I cannot fully agree with your point. (I was not talking of fs-verity, it's
a generic approach of verity approach.)
Considering we introduce a block-based verity solution for all on-disk data
to EROFS later. It means all data/compressed data and metadata are already
from a trusted source at least (like dm-verity).
Either verity->decompress or decompress->verity is safe since either
decompression algotithms or verity algorithms are _deterministic_ and
should be considered _bugfree_ therefore it should have one result.
And if you say decompression algorithm is untrusted because of bug or
somewhat, I think verity algorithm as well. In other words, if we consider
software/hardware bugs, we cannot trust any combination of results.
A advantage of verity->decompress over decompress->verity is that
the verity data is smaller than decompress->verity, so
1) we can have less I/O for most I/O patterns;
2) we can consume less CPUs.
Take a step back, there are many compression algorithm in the
user-space like apk or what ever, so the plaintext is in a
relatively speaking. We cannot consider the data to end-user is
> - Eric
next prev parent reply other threads:[~2019-08-08 6:12 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-02 22:00 [PATCH v2 0/13] Btrfs iomap Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 01/13] iomap: Use a IOMAP_COW/srcmap for a read-modify-write I/O Goldwyn Rodrigues
2019-08-03 0:39 ` Darrick J. Wong
2019-08-05 0:06 ` Dave Chinner
2019-08-02 22:00 ` [PATCH 02/13] iomap: Read page from srcmap for IOMAP_COW Goldwyn Rodrigues
2019-08-03 0:23 ` Darrick J. Wong
2019-08-04 23:52 ` Dave Chinner
2019-08-02 22:00 ` [PATCH 03/13] btrfs: Eliminate PagePrivate for btrfs data pages Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 04/13] btrfs: Add a simple buffered iomap write Goldwyn Rodrigues
2019-08-05 0:11 ` Dave Chinner
2019-08-22 15:05 ` Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 05/13] btrfs: Add CoW in iomap based writes Goldwyn Rodrigues
2019-08-05 0:13 ` Dave Chinner
2019-08-22 15:01 ` Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 06/13] btrfs: remove buffered write code made unnecessary Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 07/13] btrfs: basic direct read operation Goldwyn Rodrigues
2019-08-12 12:32 ` RITESH HARJANI
2019-08-22 15:00 ` Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 08/13] btrfs: Carve out btrfs_get_extent_map_write() out of btrfs_get_blocks_write() Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 09/13] btrfs: Rename __endio_write_update_ordered() to btrfs_update_ordered_extent() Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 10/13] iomap: use a function pointer for dio submits Goldwyn Rodrigues
2019-08-03 0:21 ` Darrick J. Wong
2019-08-05 16:08 ` Goldwyn Rodrigues
2019-08-04 23:43 ` Dave Chinner
2019-08-05 16:08 ` Goldwyn Rodrigues
2019-08-05 21:54 ` Dave Chinner
2019-08-08 4:26 ` Gao Xiang
2019-08-08 4:52 ` Gao Xiang
2019-08-08 5:49 ` Eric Biggers
2019-08-08 6:28 ` Gao Xiang [this message]
2019-08-08 8:16 ` Dave Chinner
2019-08-08 8:57 ` Gao Xiang
2019-08-08 9:29 ` Gao Xiang
2019-08-08 11:21 ` Gao Xiang
2019-08-08 13:11 ` Gao Xiang
2019-08-09 20:45 ` Matthew Wilcox
2019-08-09 23:45 ` Gao Xiang
2019-08-10 0:31 ` Eric Biggers
2019-08-10 0:50 ` Eric Biggers
2019-08-10 1:34 ` Gao Xiang
2019-08-10 1:13 ` Gao Xiang
2019-08-10 0:17 ` Eric Biggers
2019-08-02 22:00 ` [PATCH 11/13] btrfs: Use iomap_dio_rw for performing direct I/O writes Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 12/13] btrfs: Remove btrfs_dio_data and __btrfs_direct_write Goldwyn Rodrigues
2019-08-02 22:00 ` [PATCH 13/13] btrfs: update inode size during bio completion Goldwyn Rodrigues
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