From: Linus Torvalds <email@example.com> To: Andreas Gruenbacher <firstname.lastname@example.org> Cc: cluster-devel <email@example.com>, Jan Kara <firstname.lastname@example.org>, Linux Kernel Mailing List <email@example.com>, Christoph Hellwig <firstname.lastname@example.org>, David Laight <David.Laight@aculab.com>, Alexander Viro <email@example.com>, linux-fsdevel <firstname.lastname@example.org>, "email@example.com" <firstname.lastname@example.org> Subject: Re: [Ocfs2-devel] [PATCH v4 1/8] iov_iter: Introduce iov_iter_fault_in_writeable helper Date: Tue, 27 Jul 2021 10:51:38 -0700 [thread overview] Message-ID: <CAHk-=whemWRZRDDvHnesBbTo1hO2qkWkMtGUSfPvEOq7kAfouQ@mail.gmail.com> (raw) In-Reply-To: <CAHc6FU4N7vz+jfoUSa45Mr_F0Ht0_PXroWoc5UNkMgFmpKLaNw@mail.gmail.com> On Tue, Jul 27, 2021 at 4:14 AM Andreas Gruenbacher <email@example.com> wrote: > > On Tue, Jul 27, 2021 at 11:30 AM David Laight <David.Laight@aculab.com> wrote: > > > > Is it actually worth doing any more than ensuring the first byte > > of the buffer is paged in before entering the block that has > > to disable page faults? > > We definitely do want to process as many pages as we can, especially > if allocations are involved during a write. Yeah, from an efficiency standpoint, once you start walking page tables, it's probably best to just handle as much as you can. But once you get an error, I don't think it should be "everything is bad". This is a bit annoying, because while *most* users really just want that "everything is good", *some* users might just want to handle the partial success case. It's why "copy_to/from_user()" returns the number of bytes *not* written, rather than -EFAULT like get/put_user(). 99% of all users just want to know "did I write all bytes" (and then checking for a zero return is a simple and cheap verification of "everything was ok"). But then very occasionally, you hit a case where you actually want to know how much of a copy worked. It's rare, but it happens, and the read/write system calls tend to be the main user of it. And yes, the fact that "copy_to/from_user()" doesn't return an error (like get/put_user() does) has confused people many times over the years. It's annoying, but it's required by those (few) users that really do want to handle that partial case. I think this iov_iter_fault_in_readable/writeable() case should do the same. And no, it's not new to Andreas' patch. iov_iter_fault_in_readable() is doing the "everything has to be good" thing already. Which maybe implies that nobody cares about partial reads/writes. Or it's very very rare - I've seen code that handles page faults in user space, but it's admittedly been some very special CPU simulator/emulator checkpointing stuff. Linus _______________________________________________ Ocfs2-devel mailing list Ocfs2firstname.lastname@example.org https://oss.oracle.com/mailman/listinfo/ocfs2-devel
next prev parent reply other threads:[~2021-07-27 17:52 UTC|newest] Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-07-24 19:34 [Ocfs2-devel] [PATCH v4 0/8] gfs2: Fix mmap + page fault deadlocks Andreas Gruenbacher 2021-07-24 19:34 ` [Ocfs2-devel] [PATCH v4 1/8] iov_iter: Introduce iov_iter_fault_in_writeable helper Andreas Gruenbacher 2021-07-24 19:52 ` Linus Torvalds 2021-07-24 20:24 ` Al Viro 2021-07-24 20:37 ` Linus Torvalds 2021-07-24 21:38 ` Andreas Gruenbacher 2021-07-24 21:57 ` Al Viro 2021-07-24 22:06 ` Andreas Gruenbacher 2021-07-24 23:39 ` Al Viro 2021-07-27 9:30 ` David Laight 2021-07-27 11:13 ` Andreas Gruenbacher 2021-07-27 17:51 ` Linus Torvalds [this message] 2021-07-24 19:34 ` [Ocfs2-devel] [PATCH v4 2/8] gfs2: Add wrapper for iomap_file_buffered_write Andreas Gruenbacher 2021-07-24 19:34 ` [Ocfs2-devel] [PATCH v4 3/8] gfs2: Fix mmap + page fault deadlocks for buffered I/O Andreas Gruenbacher 2021-07-24 19:34 ` [Ocfs2-devel] [PATCH v4 4/8] iomap: Fix iomap_dio_rw return value for user copies Andreas Gruenbacher 2021-07-24 19:34 ` [Ocfs2-devel] [PATCH v4 5/8] iomap: Add done_before argument to iomap_dio_rw Andreas Gruenbacher 2021-07-24 19:34 ` [Ocfs2-devel] [PATCH v4 6/8] iomap: Support restarting direct I/O requests after user copy failures Andreas Gruenbacher 2021-07-24 19:34 ` [Ocfs2-devel] [PATCH v4 7/8] iov_iter: Introduce noio flag to disable page faults Andreas Gruenbacher 2021-07-24 19:34 ` [Ocfs2-devel] [PATCH v4 8/8] gfs2: Fix mmap + page fault deadlocks for direct I/O Andreas Gruenbacher
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