Re: [PATCH 1/5] vrange: Add vrange syscall and handle splitting/merging and marking vmas

[Jan Kara <jack@suse.cz>]

On Fri 21-03-14 14:17:31, John Stultz wrote:
> This patch introduces the vrange() syscall, which allows for specifying
> ranges of memory as volatile, and able to be discarded by the system.
> 
> This initial patch simply adds the syscall, and the vma handling,
> splitting and merging the vmas as needed, and marking them with
> VM_VOLATILE.
> 
> No purging or discarding of volatile ranges is done at this point.
> 
> Example man page:
> 
> NAME
> 	vrange - Mark or unmark range of memory as volatile
> 
> SYNOPSIS
> 	ssize_t vrange(unsigned_long start, size_t length,
> 			 unsigned_long mode, unsigned_long flags,
> 			 int *purged);
> 
> DESCRIPTION
> 	Applications can use vrange(2) to advise kernel that pages of
> 	anonymous mapping in the given VM area can be reclaimed without
> 	swapping (or can no longer be reclaimed without swapping).
> 	The idea is that application can help kernel with page reclaim
> 	under memory pressure by specifying data it can easily regenerate
> 	and thus kernel can discard the data if needed.
> 
> 	mode:
> 	VRANGE_VOLATILE
> 		Informs the kernel that the VM can discard in pages in
> 		the specified range when under memory pressure.
> 	VRANGE_NONVOLATILE
> 		Informs the kernel that the VM can no longer discard pages
> 		in this range.
> 
> 	flags: Currently no flags are supported.
> 
> 	purged: Pointer to an integer which will return 1 if
> 	mode == VRANGE_NONVOLATILE and any page in the affected range
> 	was purged. If purged returns zero during a mode ==
> 	VRANGE_NONVOLATILE call, it means all of the pages in the range
> 	are intact.
> 
> 	If a process accesses volatile memory which has been purged, and
> 	was not set as non volatile via a VRANGE_NONVOLATILE call, it
> 	will recieve a SIGBUS.
> 
> RETURN VALUE
> 	On success vrange returns the number of bytes marked or unmarked.
> 	Similar to write(), it may return fewer bytes then specified
> 	if it ran into a problem.
> 
> 	When using VRANGE_NON_VOLATILE, if the return value is smaller
> 	then the specified length, then the value specified by the purged
        ^^^ than
Also I'm not sure why *purged is set only if the return value is smaller
than the specified legth. Won't the interface be more logical if we set
*purged to appropriate value in all cases?

...
> diff --git a/arch/x86/syscalls/syscall_64.tbl b/arch/x86/syscalls/syscall_64.tbl
> index a12bddc..7ae3940 100644
> --- a/arch/x86/syscalls/syscall_64.tbl
> +++ b/arch/x86/syscalls/syscall_64.tbl
> @@ -322,6 +322,7 @@
>  313	common	finit_module		sys_finit_module
>  314	common	sched_setattr		sys_sched_setattr
>  315	common	sched_getattr		sys_sched_getattr
> +316	common	vrange			sys_vrange
>  
>  #
>  # x32-specific system call numbers start at 512 to avoid cache impact
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index c1b7414..a1f11da 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -117,6 +117,7 @@ extern unsigned int kobjsize(const void *objp);
>  #define VM_IO           0x00004000	/* Memory mapped I/O or similar */
>  
>  					/* Used by sys_madvise() */
> +#define VM_VOLATILE	0x00001000	/* VMA is volatile */
>  #define VM_SEQ_READ	0x00008000	/* App will access data sequentially */
>  #define VM_RAND_READ	0x00010000	/* App will not benefit from clustered reads */
>  
> diff --git a/include/linux/vrange.h b/include/linux/vrange.h
> new file mode 100644
> index 0000000..6e5331e
> --- /dev/null
> +++ b/include/linux/vrange.h
> @@ -0,0 +1,8 @@
> +#ifndef _LINUX_VRANGE_H
> +#define _LINUX_VRANGE_H
> +
> +#define VRANGE_NONVOLATILE 0
> +#define VRANGE_VOLATILE 1
> +#define VRANGE_VALID_FLAGS (0) /* Don't yet support any flags */
> +
> +#endif /* _LINUX_VRANGE_H */
> diff --git a/mm/Makefile b/mm/Makefile
> index 310c90a..20229e2 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -16,7 +16,7 @@ obj-y			:= filemap.o mempool.o oom_kill.o fadvise.o \
>  			   readahead.o swap.o truncate.o vmscan.o shmem.o \
>  			   util.o mmzone.o vmstat.o backing-dev.o \
>  			   mm_init.o mmu_context.o percpu.o slab_common.o \
> -			   compaction.o balloon_compaction.o \
> +			   compaction.o balloon_compaction.o vrange.o \
>  			   interval_tree.o list_lru.o $(mmu-y)
>  
>  obj-y += init-mm.o
> diff --git a/mm/vrange.c b/mm/vrange.c
> new file mode 100644
> index 0000000..2f8e2ce
> --- /dev/null
> +++ b/mm/vrange.c
> @@ -0,0 +1,173 @@
> +#include <linux/syscalls.h>
> +#include <linux/vrange.h>
> +#include <linux/mm_inline.h>
> +#include <linux/pagemap.h>
> +#include <linux/rmap.h>
> +#include <linux/hugetlb.h>
> +#include <linux/mmu_notifier.h>
> +#include <linux/mm_inline.h>
> +#include "internal.h"
> +
> +
> +/**
> + * do_vrange - Marks or clears VMAs in the range (start-end) as VM_VOLATILE
  If you use docbook style comments (two stars on the first line), you should
also describe arguments like we do for example in mm/memory.c.

> + *
> + * Core logic of sys_volatile. Iterates over the VMAs in the specified
> + * range, and marks or clears them as VM_VOLATILE, splitting or merging them
> + * as needed.
> + *
> + * Returns the number of bytes successfully modified.
> + *
> + * Returns error only if no bytes were modified.
> + */
> +static ssize_t do_vrange(struct mm_struct *mm, unsigned long start,
> +				unsigned long end, unsigned long mode,
> +				unsigned long flags, int *purged)
> +{
> +	struct vm_area_struct *vma, *prev;
> +	unsigned long orig_start = start;
> +	ssize_t count = 0, ret = 0;
> +
> +	down_read(&mm->mmap_sem);
> +
> +	vma = find_vma_prev(mm, start, &prev);
> +	if (vma && start > vma->vm_start)
> +		prev = vma;
> +
> +	for (;;) {
> +		unsigned long new_flags;
> +		pgoff_t pgoff;
> +		unsigned long tmp;
> +
> +		if (!vma)
> +			goto out;
> +
> +		if (vma->vm_flags & (VM_SPECIAL|VM_LOCKED|VM_MIXEDMAP|
> +					VM_HUGETLB))
> +			goto out;
> +
> +		/* We don't support volatility on files for now */
> +		if (vma->vm_file) {
> +			ret = -EINVAL;
> +			goto out;
> +		}
> +
> +		/* return ENOMEM if we're trying to mark unmapped pages */
> +		if (start < vma->vm_start) {
> +			ret = -ENOMEM;
> +			goto out;
> +		}
> +
> +		new_flags = vma->vm_flags;
> +
> +		tmp = vma->vm_end;
> +		if (end < tmp)
> +			tmp = end;
> +
> +		switch (mode) {
> +		case VRANGE_VOLATILE:
> +			new_flags |= VM_VOLATILE;
> +			break;
> +		case VRANGE_NONVOLATILE:
> +			new_flags &= ~VM_VOLATILE;
> +		}
> +
> +		pgoff = vma->vm_pgoff + ((start - vma->vm_start) >> PAGE_SHIFT);
> +		prev = vma_merge(mm, prev, start, tmp, new_flags,
> +					vma->anon_vma, vma->vm_file, pgoff,
> +					vma_policy(vma));
> +		if (prev)
> +			goto success;
> +
> +		if (start != vma->vm_start) {
> +			ret = split_vma(mm, vma, start, 1);
> +			if (ret)
> +				goto out;
> +		}
> +
> +		if (tmp != vma->vm_end) {
> +			ret = split_vma(mm, vma, tmp, 0);
> +			if (ret)
> +				goto out;
> +		}
> +
> +		prev = vma;
> +success:
> +		vma->vm_flags = new_flags;
> +
> +		/* update count to distance covered so far*/
> +		count = tmp - orig_start;
> +
> +		start = tmp;
> +		if (start < prev->vm_end)
> +			start = prev->vm_end;
> +		if (start >= end)
> +			goto out;
> +		vma = prev->vm_next;
> +	}
> +out:
> +	up_read(&mm->mmap_sem);
> +
> +	/* report bytes successfully marked, even if we're exiting on error */
> +	if (count)
> +		return count;
> +
> +	return ret;
> +}
> +
> +
> +/**
> + * sys_vrange - Marks specified range as volatile or non-volatile.
> + *
> + * Validates the syscall inputs and calls do_vrange(), then copies the
> + * purged flag back out to userspace.
> + *
> + * Returns the number of bytes successfully modified.
> + * Returns error only if no bytes were modified.
> + */
> +SYSCALL_DEFINE5(vrange, unsigned long, start, size_t, len, unsigned long, mode,
> +			unsigned long, flags, int __user *, purged)
> +{
> +	unsigned long end;
> +	struct mm_struct *mm = current->mm;
> +	ssize_t ret = -EINVAL;
> +	int p = 0;
> +
> +	if (flags & ~VRANGE_VALID_FLAGS)
> +		goto out;
> +
> +	if (start & ~PAGE_MASK)
> +		goto out;
> +
> +	len &= PAGE_MASK;
> +	if (!len)
> +		goto out;
> +
> +	end = start + len;
> +	if (end < start)
> +		goto out;
> +
> +	if (start >= TASK_SIZE)
> +		goto out;
> +
> +	if (purged) {
> +		/* Test pointer is valid before making any changes */
> +		if (put_user(p, purged))
> +			return -EFAULT;
> +	}
> +
> +	ret = do_vrange(mm, start, end, mode, flags, &p);
> +
> +	if (purged) {
> +		if (put_user(p, purged)) {
> +			/*
> +			 * This would be bad, since we've modified volatilty
> +			 * and the change in purged state would be lost.
> +			 */
> +			WARN_ONCE(1, "vrange: purge state possibly lost\n");
  I think this can happen when the application has several threads and
vrange() in one thread races with munmap() in another thread. So
WARN_ONCE() doesn't look appropriate (kernel shouldn't spew warnings about
application programming bugs)... I'd just return -EFAULT. I know
information will be lost but userspace is doing something utterly stupid.

> +		}
> +	}
> +
> +out:
> +	return ret;
> +}

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR