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CpVd5GklZAfKT2BsTLA7ACCA2m2oEiJTLccD5CSlZUoIFK2iA0kSKX8eyuSzDkGpDZFd4cVRLiKBgbcOASROqV9EoC91uUXEbA7SdqPln7/D1JvsnrJ8v2Z2sRJYV2XX42b0Wkit6OGhtqWLSkUZxhvAFjE/otI5nyALqZXRUPbCR5jUrEKkPjHzT2nsLltOcvhYLMjDeBdMct5pA6+/TiHz15Vv7it0RH2InbgeTIuRMdCf2xoUOsj/wxv7zWh/J5XV6hY1kxu7VPSWp1P63CdJ+bfEQHlne5O2xzUeXtxR6KDyjLUGUykBxLcSFz+QtRvAVcsUdGsEl2ZX6h1fzv7+q2w== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: --=_MailMate_377682A3-E234-490E-9B00-DAC3848916AD_= Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On 14 Mar 2024, at 5:03, Jan Kara wrote: > On Fri 08-03-24 05:17:46, Barry Song wrote: >> On Fri, Mar 8, 2024 at 5:06=E2=80=AFAM Jared Hulbert wrote: >>> >>> On Thu, Mar 7, 2024 at 9:35=E2=80=AFAM Jan Kara wrote:= >>>> >>>> Well, but then if you fill in space of a particular order and need t= o swap >>>> out a page of that order what do you do? Return ENOSPC prematurely? >>>> >>>> Frankly as I'm reading the discussions here, it seems to me you are = trying >>>> to reinvent a lot of things from the filesystem space :) Like block >>>> allocation with reasonably efficient fragmentation prevention, trans= parent >>>> data compression (zswap), hierarchical storage management (i.e., mov= ing >>>> data between different backing stores), efficient way to get from >>>> VMA+offset to the place on disk where the content is stored. Sure yo= u still >>>> don't need a lot of things modern filesystems do like permissions,> = directory structure (or even more complex namespacing stuff), all the stu= ff >>>> achieving fs consistency after a crash, etc. But still what you need= is a >>>> notable portion of what filesystems do. >>>> >>>> So maybe it would be time to implement swap as a proper filesystem? = Or even >>>> better we could think about factoring out these bits out of some exi= sting >>>> filesystem to share code? >>> >>> Yes. Thank you. I've been struggling to communicate this. >>> >>> I'm thinking you can just use existing filesystems as a first step >>> with a modest glue layer. See the branch of this thread where I'm >>> babbling on to Chris about this. >>> >>> "efficient way to get from VMA+offset to place on the disk where >>> content is stored" >>> You mean treat swapped pages like they were mmap'ed files and use the= >>> same code paths? How big of a project is that? That seems either >>> deceptively easy or really hard... I've been away too long and was >>> never really good enough to have a clear vision of the scale. >> >> I don't understand why we need this level of complexity. All we need t= o >> know are the offsets during pageout. After that, the large folio is >> destroyed, and all offsets are stored in page table entries (PTEs) or = xa. >> Swap-in doesn't depend on a complex file system; it can make its own >> decision on how to swap-in based on the values it reads from PTEs. > > Well, but once compression chimes in (like with zswap) or if you need t= o > perform compaction on swap space and move swapped out data, things aren= 't > that simple anymore, are they? So as I was reading this thread I had th= e > impression that swap complexity is coming close to a complexity of a > (relatively simple) filesystem so I was brainstorming about possibility= of > sharing some code between filesystems and swap... I think all the complexity comes from that we want to preserve folios as a whole, thus need to handle fragmentation issues. But Barry=E2=80=99s ap= proach is trying to get us away from it. The downside is what you mentioned about compression, since 64KB should give better compression ratio than 4KB. For swap without compression, we probably can use Barry=E2=80=99s approach to keep everything simple, just split all folios when they go into swap, but I am not sure about if there is disk throughput loss. For zswap, there will be design tradeoff between better compression ratio= and complexity. Best Regards, Yan, Zi --=_MailMate_377682A3-E234-490E-9B00-DAC3848916AD_= Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename=signature.asc Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQJDBAEBCgAtFiEEh7yFAW3gwjwQ4C9anbJR82th+ooFAmZGIGgPHHppeUBudmlk aWEuY29tAAoJEJ2yUfNrYfqKHSIQAJo/Zh7sD2foRvoZblWOton0GuHsOvS6r3Q4 Uhh4vP6E3FqpdPci+2TyvdK7nFVC41i+H8/dfQaqezDASkx+OD6kdHnGoPiV2fIx oulzg6n/tmPCDJryGSrMfLyx0+m0jliIDdCqexX0yqiTt/+IU4y8JvdQeaGnxFVf Nh5P46269xTPXSFgm7ELYUrwBHINZ46kWu3CdVnbexfQe4ATkhH9EGUBNjcIQBn7 pGbBoY3vXmQW0K4LPVrXDMtERffGeie9FECvAbpB9/0eWD2l+AaJikm8WMKNJNZI XPQoGYXZmmKQ7bLsHeDVGUCm2iECX58MahAX71+/jYtgzgdzVCVo0HPV1iK2PGLS 7koYCsyVuAnVieG4l1vmd0OW+ljqqU8M1lY/25P2IVNcHMTWq0QEwzL04UAJ/XJl kA4uFkiLA0FbnEENJTkOUqPP9l0PDA0cxF4QyRZErf8AfUWqb9HPslitOGZXhqny W2NDj7M336ZP/7ERczmy5B008JxIY/XIHSntdgzbK1z5SDxqAYmXvK7N8bNQNeRZ 5M0H06w1pi+4OJMP+NHrFU+5wMwdVTN6MQ0yzDixDckYUtd8yJI7JzkvRgCq+W7R RoaPeba98euXPZTsD0qww+kNNRZIZecv/cG5gygVLrXovuwBbyZ4bh6ZFm6g68Q6 G2vt776+ =creC -----END PGP SIGNATURE----- --=_MailMate_377682A3-E234-490E-9B00-DAC3848916AD_=--