Messages in this thread |  | | Date | Wed, 29 Jun 2011 02:38:20 -0700 | | Subject | Re: [PATCH 1/2] mm: Remove use of ALLOW_RETRY when RETRY_NOWAIT is set | | From | Michel Lespinasse <> |
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On Tue, Jun 28, 2011 at 9:47 AM, Steven Rostedt <rostedt@goodmis.org> wrote: > From: Steven Rostedt <srostedt@redhat.com> > > The only user of FAULT_FLAG_RETRY_NOWAIT also sets the > FAULT_FLAG_ALLOW_RETRY flag. This makes the check in the > __lock_page_or_retry redundant as it checks the RETRY_NOWAIT > just after checking ALLOW_RETRY and then returns if it is > set. The FAULT_FLAG_ALLOW_RETRY does not make any other > difference in this path. > > Setting both and then ignoring one is quite confusing, > especially since this code has very subtle locking issues > when it comes to the mmap_sem. > > Only set the RETRY_WAIT flag and have that do the necessary > work instead of confusing reviewers of this code by setting > ALLOW_RETRY and not releasing the mmap_sem. > > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -151,8 +151,8 @@ extern pgprot_t protection_map[16]; > #define FAULT_FLAG_WRITE 0x01 /* Fault was a write access */ > #define FAULT_FLAG_NONLINEAR 0x02 /* Fault was via a nonlinear mapping */ > #define FAULT_FLAG_MKWRITE 0x04 /* Fault was mkwrite of existing pte */ > -#define FAULT_FLAG_ALLOW_RETRY 0x08 /* Retry fault if blocking */ > -#define FAULT_FLAG_RETRY_NOWAIT 0x10 /* Don't drop mmap_sem and wait when retrying */ > +#define FAULT_FLAG_ALLOW_RETRY 0x08 /* Retry fault if blocking (drops mmap_sem) */ > +#define FAULT_FLAG_RETRY_NOWAIT 0x10 /* Wait when retrying (don't drop mmap_sem) */
You want to say "DONT wait when retrying" here...
Also - you argued higher up that having both flags set at once is confusing, but I find it equally confusing to pass a flag to specify you don't want to wait on retry if the flag that allows retry is not set. I think the confusion comes from the way the nowait semantics got bolted on the retry code for virtualization, even though (if I understand the virtualization use case correctly) they dont actually want to retry there, they just want to give up without blocking.
Would the following proposal make more sense to you ?
FAULT_FLAG_ALLOW_ASYNC: allow returning a VM_FAULT_ASYNC error code if the page can't be obtained immediately (major fault). FAULT_FLAG_ASYNC_WAIT: before returning VM_FAULT_ASYNC, drop the mmap_sem and wait for major fault to complete.
existing uses of FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT become FAULT_FLAG_ASYNC existing uses of FAULT_FLAG_ALLOW_RETRY alone become FAULT_FLAG_ASYNC | FAULT_FLAG_ASYNC_WAIT existing uses of VM_FAULT_RETRY become VM_FAULT_ASYNC
This may also help your documentation proposal since the flags would now work together rather than having one be an exception to the other.
-- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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