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SubjectRe: [PATCH] kiobuf/rawio fixes for 2.4.0-test10-pre6
On Fri, Oct 27, 2000 at 02:23:04PM -0700, Linus Torvalds wrote:
>
> [...]
>
> That solution, btw, might be as simple as just saying:
>
> - raw IO is based on physical pages, and the COW mapping crated by
> fork() may cause the changes to be visibile to either child or parent
> or both, depending on usage patterns to the page in question. For
> repeatable behaviour, do not have outstanding direct IO in progress
> over a fork().
>
> Ie, just _document_ it. It's not _wrong_, it can just be surprising (but
> it is actually entirely straightforward and sane if you just look at it
> the right way).

Ok, here is an updated patch witout that change, but instead with a little
piece of kiobuf documentation that does document this and other things
related to kiobufs.

Christoph

--
Always remember that you are unique. Just like everyone else.


--- linux.orig/drivers/char/raw.c Thu Oct 19 13:21:24 2000
+++ linux/drivers/char/raw.c Sun Oct 29 20:55:43 2000
@@ -277,8 +277,11 @@

if ((*offp & sector_mask) || (size & sector_mask))
return -EINVAL;
- if ((*offp >> sector_bits) > limit)
+ if ((*offp >> sector_bits) > limit) {
+ if (size)
+ return -ENXIO;
return 0;
+ }

/*
* We'll just use one kiobuf
--- linux.orig/fs/buffer.c Fri Oct 27 12:28:40 2000
+++ linux/fs/buffer.c Sun Oct 29 20:55:43 2000
@@ -1924,6 +1924,8 @@

spin_unlock(&unused_list_lock);

+ if (!iosize)
+ return -EIO;
return iosize;
}

--- linux.orig/mm/memory.c Fri Oct 27 12:28:42 2000
+++ linux/mm/memory.c Sun Oct 29 20:56:09 2000
@@ -382,9 +382,12 @@


/*
- * Do a quick page-table lookup for a single page.
+ * Do a quick page-table lookup for a single page. We have already verified
+ * access type, and done a fault in. But, kswapd might have stolen the page
+ * in the meantime. Return an indication of whether we should retry the fault
+ * in. Writability test is superfluous but conservative.
*/
-static struct page * follow_page(unsigned long address)
+static struct page * follow_page(unsigned long address, int writeacc, int * ret)
{
pgd_t *pgd;
pmd_t *pmd;
@@ -393,10 +396,15 @@
pmd = pmd_offset(pgd, address);
if (pmd) {
pte_t * pte = pte_offset(pmd, address);
- if (pte && pte_present(*pte))
+ if (pte && pte_present(*pte)) {
+ if (writeacc && !pte_write(*pte))
+ goto retry;
return pte_page(*pte);
+ }
}
-
+
+retry:
+ *ret = 1;
return NULL;
}

@@ -428,7 +436,8 @@
struct page * map;
int i;
int datain = (rw == READ);
-
+ int failed;
+
/* Make sure the iobuf is not already mapped somewhere. */
if (iobuf->nr_pages)
return -EINVAL;
@@ -467,18 +476,22 @@
}
if (((datain) && (!(vma->vm_flags & VM_WRITE))) ||
(!(vma->vm_flags & VM_READ))) {
- err = -EACCES;
goto out_unlock;
}
}
+
+faultin:
if (handle_mm_fault(current->mm, vma, ptr, datain) <= 0)
goto out_unlock;
spin_lock(&mm->page_table_lock);
- map = follow_page(ptr);
- if (!map) {
+ map = follow_page(ptr, datain, &failed);
+ if (failed) {
+ /*
+ * Page got stolen before we could lock it down.
+ * Retry.
+ */
spin_unlock(&mm->page_table_lock);
- dprintk (KERN_ERR "Missing page in map_user_kiobuf\n");
- goto out_unlock;
+ goto faultin;
}
map = get_page_map(map);
if (map)
diff -uNr linux.orig/Documentation/kiobuf.txt linux/Documentation/kiobuf.txt
--- linux.orig/Documentation/kiobuf.txt Thu Jan 1 01:00:00 1970
+++ linux/Documentation/kiobuf.txt Sun Oct 29 21:38:20 2000
@@ -0,0 +1,100 @@
+ Abstract Kernel IO Buffers
+ Under Linux
+
+ Christoph Hellwig <hch@caldera.de>
+
+
+This document describes the kiobuf concept used in the Linux Kernel
+IO/memory subsystem. It describes it's usages, functions working
+with kernel IO buffers and show some examples for kiobuf usage.
+
+
+The main reason for implementing kernel IO buffers (by Stephen Tweedie)
+was the lack of raw devices support in Linux kernels <= 2.2. Raw devices
+are the character devices that AT&T derived UNIX version implement to
+allow character based uncached access to mass storage devices. In
+Linux kernels <= 2.2 all blockdevice IO goes either through the buffer-
+or pagecache, so that applications like databases cannot get full
+control over their data.
+
+The solution in Linux 2.3 an higher is that the new raw devices driver
+locks down the virtual memory it gets passed by the ->read and ->write
+methods and does physical page io on them, bypassing the caches.
+NOTE: the physical memory referenced by kiobufs does - unlike nearly
+everything else in the Linux memory managment - not have reasonable COW
+semenantics. So don't even try to fork when doing rawio or using
+user-space memory in kiobufs in an other way.
+
+
+To use iobufs in this way you need to allocate one or more kiobufs (an
+array of kiobufs is called kiovec - do not confuse those with BSD iovecs).
+
+ err = alloc_kiovec (count, iovec);
+
+This allocates the memory for the wanted number of kiobufs (and adds them
+to a cache) and initalizes some variables - in an OO-language this would be
+the constructor. Then you force the virtual memory to faulted in and locked
+in physical memory and reference it by the kiobuf. (NOTE: this must be done
+for each iobuf, not for the whole iovec).
+
+ err = map_user_kiobuf (rw, iobuf, address, len);
+
+After that you request IO against the wanted device. For the case of
+raw devices where IO should be requested against a blockdevice, there
+is a function in fs/buffer.c that does exactly this. (the parameter
+'blocks' is an array of the block numbers the IO should be requested
+against)
+
+ err = brw_kiovec (rw, count, iovec, dev, blocks, sector_size);
+
+After the IO for this iobuf is done, unmap the virtual memory.
+
+ unmap_kiobuf (iobuf);
+
+And when we are finished with the iovev, free it.
+
+ free_kiovec (count, iovec);
+
+
+Locking down user memory and doing mass storage device IO with it is not
+the only purpose of kiobufs. Another use for kiobufs is allowing
+user-space mmaping dma memory, e.g in sound drivers. To do so you
+need to lock-down kernel virtual memory and refernece it using kiobufs.
+The code that does exactly this is not yet in the kernel - get Stephen
+Tweedie's kiobuf patchset if you want to use this.
+
+
+In the long term it looks like all blockdev IO will be done using
+kiobufs. In the SGI XFS tree there is code that allows passing kiovecs
+to the individual low-level block drivers. There are lots of advantages
+of doing it this way: the page cache doesn't need to fit the outstanding
+io into lots of bufferheads, passing each bufferhead to ll_rw_block()
+where the elevator merges some of them together for better device usage
+and submits them to the drivers. Instead the cache locks down the pages
+and submits the kiovec to the low-level driver. The lowlevel driver knows
+better how the request should be splitted for dmaing or whatever. On the
+other hand software RAID or LVM get more complicated: instead of just
+doing block-remapping they must split the kiobufs and - in case of LVM -
+find ways to do efficient IO on continguos areas.
+
+
+
+References:
+
+ Linux Kernel Sourcecode
+ (fs/buffer.c, fs/iobufs.c, mm/memory.c, drivers/char/raw.c)
+
+ SGI XFS for Linux
+ (http://oss.sgi.com/projects/linux-xfs/)
+
+ Stephen Tweedies kiobuf patchset
+ (ftp://ftp.linux.org.uk/pub/linux/sct/fs/raw-io/)
+
+ Linux MM mailinglist
+ (http://humbolt.geo.uu.nl/Linux-MM/linux-mm.html)
+
+
+Thanks to Arjan van de Ven, Daniel Phillips and Marcelo Tosatti for
+proofreading this document and giving usefull hints.
+
+$Id: kiobuf.txt,v 1.2 2000/10/29 20:37:54 hch Exp hch $
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