Re: [PATCH 04/11] filemap: add FGP_CREAT_ONLY

[Vlastimil Babka]

On 4/25/24 7:52 AM, Paolo Bonzini wrote:
> On 4/4/24 20:50, Paolo Bonzini wrote:
>> KVM would like to add a ioctl to encrypt and install a page into private
>> memory (i.e. into a guest_memfd), in preparation for launching an
>> encrypted guest.
>> 
>> This API should be used only once per page (unless there are failures),
>> so we want to rule out the possibility of operating on a page that is
>> already in the guest_memfd's filemap.  Overwriting the page is almost
>> certainly a sign of a bug, so we might as well forbid it.
>> 
>> Therefore, introduce a new flag for __filemap_get_folio (to be passed
>> together with FGP_CREAT) that allows *adding* a new page to the filemap
>> but not returning an existing one.
>> 
>> An alternative possibility would be to force KVM users to initialize
>> the whole filemap in one go, but that is complicated by the fact that
>> the filemap includes pages of different kinds, including some that are
>> per-vCPU rather than per-VM.  Basically the result would be closer to
>> a system call that multiplexes multiple ioctls, than to something
>> cleaner like readv/writev.
>> 
>> Races between callers that pass FGP_CREAT_ONLY are uninteresting to
>> the filemap code: one of the racers wins and one fails with EEXIST,
>> similar to calling open(2) with O_CREAT|O_EXCL.  It doesn't matter to
>> filemap.c if the missing synchronization is in the kernel or in userspace,
>> and in fact it could even be intentional.  (In the case of KVM it turns
>> out that a mutex is taken around these calls for unrelated reasons,
>> so there can be no races.)
>> 
>> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
>> Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
>> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> 
> Matthew, are your objections still valid or could I have your ack?

So per the sub-thread on PATCH 09/11, IIUC this is now moot, right?

Vlastimil

> Thanks,
> 
> Paolo
> 
>> ---
>>   include/linux/pagemap.h | 2 ++
>>   mm/filemap.c            | 4 ++++
>>   2 files changed, 6 insertions(+)
>> 
>> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
>> index f879c1d54da7..a8c0685e8c08 100644
>> --- a/include/linux/pagemap.h
>> +++ b/include/linux/pagemap.h
>> @@ -587,6 +587,7 @@ pgoff_t page_cache_prev_miss(struct address_space *mapping,
>>    * * %FGP_CREAT - If no folio is present then a new folio is allocated,
>>    *   added to the page cache and the VM's LRU list.  The folio is
>>    *   returned locked.
>> + * * %FGP_CREAT_ONLY - Fail if a folio is present
>>    * * %FGP_FOR_MMAP - The caller wants to do its own locking dance if the
>>    *   folio is already in cache.  If the folio was allocated, unlock it
>>    *   before returning so the caller can do the same dance.
>> @@ -607,6 +608,7 @@ typedef unsigned int __bitwise fgf_t;
>>   #define FGP_NOWAIT		((__force fgf_t)0x00000020)
>>   #define FGP_FOR_MMAP		((__force fgf_t)0x00000040)
>>   #define FGP_STABLE		((__force fgf_t)0x00000080)
>> +#define FGP_CREAT_ONLY		((__force fgf_t)0x00000100)
>>   #define FGF_GET_ORDER(fgf)	(((__force unsigned)fgf) >> 26)	/* top 6 bits */
>>   
>>   #define FGP_WRITEBEGIN		(FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE)
>> diff --git a/mm/filemap.c b/mm/filemap.c
>> index 7437b2bd75c1..e7440e189ebd 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -1863,6 +1863,10 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
>>   		folio = NULL;
>>   	if (!folio)
>>   		goto no_page;
>> +	if (fgp_flags & FGP_CREAT_ONLY) {
>> +		folio_put(folio);
>> +		return ERR_PTR(-EEXIST);
>> +	}
>>   
>>   	if (fgp_flags & FGP_LOCK) {
>>   		if (fgp_flags & FGP_NOWAIT) {
>