Re: [RFC PATCH 2/8] mm: Add PG_atomic

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

Matthew Wilcox <willy@infradead.org> writes:

> On Fri, Nov 14, 2025 at 10:30:09AM +0530, Ritesh Harjani wrote:
>> Matthew Wilcox <willy@infradead.org> writes:
>> 
>> > On Wed, Nov 12, 2025 at 04:36:05PM +0530, Ojaswin Mujoo wrote:
>> >> From: John Garry <john.g.garry@oracle.com>
>> >> 
>> >> Add page flag PG_atomic, meaning that a folio needs to be written back
>> >> atomically. This will be used by for handling RWF_ATOMIC buffered IO
>> >> in upcoming patches.
>> >
>> > Page flags are a precious resource.  I'm not thrilled about allocating one
>> > to this rather niche usecase.  Wouldn't this be more aptly a flag on the
>> > address_space rather than the folio?  ie if we're doing this kind of write
>> > to a file, aren't most/all of the writes to the file going to be atomic?
>> 
>> As of today the atomic writes functionality works on the per-write
>> basis (given it's a per-write characteristic). 
>> 
>> So, we can have two types of dirty folios sitting in the page cache of
>> an inode. Ones which were done using atomic buffered I/O flag
>> (RWF_ATOMIC) and the other ones which were non-atomic writes. Hence a
>> need of a folio flag to distinguish between the two writes.
>
> I know, but is this useful?  AFAIK, the files where Postgres wants to
> use this functionality are the log files, and all writes to the log
> files will want to use the atomic functionality.  What's the usecase
> for "I want to mix atomic and non-atomic buffered writes to this file"?

Actually this goes back to the design of how we added support of atomic
writes during DIO. So during the initial design phase we decided that
this need not be a per-inode attribute or an open flag, but this is a
per write I/O characteristic.

So as per the current design, we don't have any open flag or a
persistent inode attribute which says kernel should permit _only_ atomic
writes I/O to this file. Instead what we support today is DIO atomic
writes using RWF_ATOMIC flag in pwritev2 syscall.

Having said that there can be several policy decision that could still be
discussed e.g. make sure any previous dirty data is flushed to disk when a
buffered atomic write request is made to an inode. 
Maybe that would allow us to just keep a flag at the address space level
because we would never have a mix of atomic and non-atomic page cache
pages.

IMO, I agree that folio flag is a scarce resource, but I guess the
initial goal of this patch series is mainly to discuss the initial
design of the core feature i.e. how buffered atomic writes should look
in Linux kernel. I agree and point taken that we should be careful with
using folio flags, but let's see how the design shapes up maybe? - that
will help us understand whether a folio flag is really required or maybe
an address space flag would do. 

-ritesh