Re: [PATCH v3 04/28] mm: allow VM_FAULT_RETRY for multiple times

[Jerome Glisse]

On Wed, Mar 20, 2019 at 10:06:18AM +0800, Peter Xu wrote:
> The idea comes from a discussion between Linus and Andrea [1].
> 
> Before this patch we only allow a page fault to retry once.  We
> achieved this by clearing the FAULT_FLAG_ALLOW_RETRY flag when doing
> handle_mm_fault() the second time.  This was majorly used to avoid
> unexpected starvation of the system by looping over forever to handle
> the page fault on a single page.  However that should hardly happen,
> and after all for each code path to return a VM_FAULT_RETRY we'll
> first wait for a condition (during which time we should possibly yield
> the cpu) to happen before VM_FAULT_RETRY is really returned.
> 
> This patch removes the restriction by keeping the
> FAULT_FLAG_ALLOW_RETRY flag when we receive VM_FAULT_RETRY.  It means
> that the page fault handler now can retry the page fault for multiple
> times if necessary without the need to generate another page fault
> event.  Meanwhile we still keep the FAULT_FLAG_TRIED flag so page
> fault handler can still identify whether a page fault is the first
> attempt or not.
> 
> Then we'll have these combinations of fault flags (only considering
> ALLOW_RETRY flag and TRIED flag):
> 
>   - ALLOW_RETRY and !TRIED:  this means the page fault allows to
>                              retry, and this is the first try
> 
>   - ALLOW_RETRY and TRIED:   this means the page fault allows to
>                              retry, and this is not the first try
> 
>   - !ALLOW_RETRY and !TRIED: this means the page fault does not allow
>                              to retry at all
> 
>   - !ALLOW_RETRY and TRIED:  this is forbidden and should never be used
> 
> In existing code we have multiple places that has taken special care
> of the first condition above by checking against (fault_flags &
> FAULT_FLAG_ALLOW_RETRY).  This patch introduces a simple helper to
> detect the first retry of a page fault by checking against
> both (fault_flags & FAULT_FLAG_ALLOW_RETRY) and !(fault_flag &
> FAULT_FLAG_TRIED) because now even the 2nd try will have the
> ALLOW_RETRY set, then use that helper in all existing special paths.
> One example is in __lock_page_or_retry(), now we'll drop the mmap_sem
> only in the first attempt of page fault and we'll keep it in follow up
> retries, so old locking behavior will be retained.
> 
> This will be a nice enhancement for current code [2] at the same time
> a supporting material for the future userfaultfd-writeprotect work,
> since in that work there will always be an explicit userfault
> writeprotect retry for protected pages, and if that cannot resolve the
> page fault (e.g., when userfaultfd-writeprotect is used in conjunction
> with swapped pages) then we'll possibly need a 3rd retry of the page
> fault.  It might also benefit other potential users who will have
> similar requirement like userfault write-protection.
> 
> GUP code is not touched yet and will be covered in follow up patch.
> 
> Please read the thread below for more information.
> 
> [1] https://lkml.org/lkml/2017/11/2/833
> [2] https://lkml.org/lkml/2018/12/30/64
> 
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Suggested-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>

Reviewed-by: Jérôme Glisse <jglisse@xxxxxxxxxx>

A minor comment suggestion below but it can be fix in a followup patch.

[...]

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 80bb6408fe73..f73dbc4a1957 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -336,16 +336,52 @@ extern unsigned int kobjsize(const void *objp);
>   */
>  extern pgprot_t protection_map[16];
>  
> +/*
> + * About FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_TRIED: we can specify whether we
> + * would allow page faults to retry by specifying these two fault flags
> + * correctly.  Currently there can be three legal combinations:
> + *
> + * (a) ALLOW_RETRY and !TRIED:  this means the page fault allows retry, and
> + *                              this is the first try
> + *
> + * (b) ALLOW_RETRY and TRIED:   this means the page fault allows retry, and
> + *                              we've already tried at least once
> + *
> + * (c) !ALLOW_RETRY and !TRIED: this means the page fault does not allow retry
> + *
> + * The unlisted combination (!ALLOW_RETRY && TRIED) is illegal and should never
> + * be used.  Note that page faults can be allowed to retry for multiple times,
> + * in which case we'll have an initial fault with flags (a) then later on
> + * continuous faults with flags (b).  We should always try to detect pending
> + * signals before a retry to make sure the continuous page faults can still be
> + * interrupted if necessary.
> + */
> +
>  #define FAULT_FLAG_WRITE	0x01	/* Fault was a write access */
>  #define FAULT_FLAG_MKWRITE	0x02	/* Fault was mkwrite of existing pte */
>  #define FAULT_FLAG_ALLOW_RETRY	0x04	/* Retry fault if blocking */
>  #define FAULT_FLAG_RETRY_NOWAIT	0x08	/* Don't drop mmap_sem and wait when retrying */
>  #define FAULT_FLAG_KILLABLE	0x10	/* The fault task is in SIGKILL killable region */
> -#define FAULT_FLAG_TRIED	0x20	/* Second try */
> +#define FAULT_FLAG_TRIED	0x20	/* We've tried once */
>  #define FAULT_FLAG_USER		0x40	/* The fault originated in userspace */
>  #define FAULT_FLAG_REMOTE	0x80	/* faulting for non current tsk/mm */
>  #define FAULT_FLAG_INSTRUCTION  0x100	/* The fault was during an instruction fetch */
>  
> +/*
> + * Returns true if the page fault allows retry and this is the first
> + * attempt of the fault handling; false otherwise.  This is mostly
> + * used for places where we want to try to avoid taking the mmap_sem
> + * for too long a time when waiting for another condition to change,
> + * in which case we can try to be polite to release the mmap_sem in
> + * the first round to avoid potential starvation of other processes
> + * that would also want the mmap_sem.
> + */

You should be using kernel function documentation style above.

> +static inline bool fault_flag_allow_retry_first(unsigned int flags)
> +{
> +	return (flags & FAULT_FLAG_ALLOW_RETRY) &&
> +	    (!(flags & FAULT_FLAG_TRIED));
> +}
> +