Re: [PATCH v2 1/3] mm: introduce new .mmap_prepare() file callback

[David Hildenbrand <david@redhat.com>]

On 13.05.25 11:32, Lorenzo Stoakes wrote:
> On Tue, May 13, 2025 at 11:01:41AM +0200, David Hildenbrand wrote:
>> On 09.05.25 14:13, Lorenzo Stoakes wrote:
>>> Provide a means by which drivers can specify which fields of those
>>> permitted to be changed should be altered to prior to mmap()'ing a
>>> range (which may either result from a merge or from mapping an entirely new
>>> VMA).
>>>
>>> Doing so is substantially safer than the existing .mmap() calback which
>>> provides unrestricted access to the part-constructed VMA and permits
>>> drivers and file systems to do 'creative' things which makes it hard to
>>> reason about the state of the VMA after the function returns.
>>>
>>> The existing .mmap() callback's freedom has caused a great deal of issues,
>>> especially in error handling, as unwinding the mmap() state has proven to
>>> be non-trivial and caused significant issues in the past, for instance
>>> those addressed in commit 5de195060b2e ("mm: resolve faulty mmap_region()
>>> error path behaviour").
>>>
>>> It also necessitates a second attempt at merge once the .mmap() callback
>>> has completed, which has caused issues in the past, is awkward, adds
>>> overhead and is difficult to reason about.
>>>
>>> The .mmap_prepare() callback eliminates this requirement, as we can update
>>> fields prior to even attempting the first merge. It is safer, as we heavily
>>> restrict what can actually be modified, and being invoked very early in the
>>> mmap() process, error handling can be performed safely with very little
>>> unwinding of state required.
>>>
>>> The .mmap_prepare() and deprecated .mmap() callbacks are mutually
>>> exclusive, so we permit only one to be invoked at a time.
>>>
>>> Update vma userland test stubs to account for changes.
>>>
>>> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>>> ---
>>>    include/linux/fs.h               | 25 ++++++++++++
>>>    include/linux/mm_types.h         | 24 +++++++++++
>>>    mm/memory.c                      |  3 +-
>>>    mm/mmap.c                        |  2 +-
>>>    mm/vma.c                         | 68 +++++++++++++++++++++++++++++++-
>>>    tools/testing/vma/vma_internal.h | 66 ++++++++++++++++++++++++++++---
>>>    6 files changed, 180 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>>> index 016b0fe1536e..e2721a1ff13d 100644
>>> --- a/include/linux/fs.h
>>> +++ b/include/linux/fs.h
>>> @@ -2169,6 +2169,7 @@ struct file_operations {
>>>    	int (*uring_cmd)(struct io_uring_cmd *ioucmd, unsigned int issue_flags);
>>>    	int (*uring_cmd_iopoll)(struct io_uring_cmd *, struct io_comp_batch *,
>>>    				unsigned int poll_flags);
>>> +	int (*mmap_prepare)(struct vm_area_desc *);
>>>    } __randomize_layout;
>>>    /* Supports async buffered reads */
>>> @@ -2238,11 +2239,35 @@ struct inode_operations {
>>>    	struct offset_ctx *(*get_offset_ctx)(struct inode *inode);
>>>    } ____cacheline_aligned;
>>> +/* Did the driver provide valid mmap hook configuration? */
>>> +static inline bool file_has_valid_mmap_hooks(struct file *file)
>>> +{
>>> +	bool has_mmap = file->f_op->mmap;
>>> +	bool has_mmap_prepare = file->f_op->mmap_prepare;
>>> +
>>> +	/* Hooks are mutually exclusive. */
>>> +	if (WARN_ON_ONCE(has_mmap && has_mmap_prepare))
>>> +		return false;
>>> +	if (WARN_ON_ONCE(!has_mmap && !has_mmap_prepare))
>>> +		return false;
>>> +
>>> +	return true;
>>> +}
>>
>> So, if neither is set, it's also an invalid setting, understood.
>>
>> So we want XOR.
>>
>>
>>
>> const bool has_mmap = file->f_op->mmap;
>> const bool has_mmap_prepare = file->f_op->mmap_prepare;
>> const bool mutual_exclusive = has_mmap ^ has_mmap_prepare;
>>
>> WARN_ON_ONCE(!mutual_exclusive)
>> return mutual_exclusive;
> 
> Yeah I did consider xor like this but I've always found it quite confusing
> in this kind of context, honestly.

With the local variable I think it's quite helpful (no need for a 
comment :P ).

> 
> In a way I think it's a bit easier spelt out as it is now. But happy to
> change if you feel strongly about it? :)

Certainly not strongly! :)

-- 
Cheers,

David / dhildenb