linux-kernel - Re: [RFC][PATCH 0/4] Enhanced file stat system call

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Message-Id: <3852DE26-42A6-4AFF-AA9C-08C4E9837511@dilger.ca>
Date:   Thu, 17 Nov 2016 19:30:25 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     "J. Bruce Fields" <bfields@...ldses.org>
Cc:     David Howells <dhowells@...hat.com>,
        One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 0/4] Enhanced file stat system call


> On Nov 17, 2016, at 1:00 PM, J. Bruce Fields <bfields@...ldses.org> wrote:
> 
> On Thu, Nov 17, 2016 at 04:45:45PM +0000, David Howells wrote:
>> One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk> wrote:
>> 
>>>> (2) Lightweight stat (AT_STATX_DONT_SYNC): Ask for just those details of
>>>>     interest, and allow a network fs to approximate anything not of
>>>>     interest, without going to the server.
>>>> 
>>>> (3) Heavyweight stat (AT_STATX_FORCE_SYNC): Force a network fs to flush
>>>>     buffers and go to the server, even if it thinks its cached attributes
>>>>     are up to date.
>>> 
>>> That seems an odd way to do it. Wouldn't it be cleaner and more flexible
>>> to give a timestamp of the oldest time you consider acceptable (and
>>> obviously passing 0 indicates whatever you have)
>> 
>> Perhaps, though adding 6-argument syscalls is apparently frowned upon.
>> 
>>>> Note that no lstat() equivalent is required as that can be implemented
>>>> through statx() with atflag == 0.  There is also no fstat() equivalent as
>>>> that can be implemented through statx() with filename == NULL and the
>>>> relevant fd passed as dfd.
>>> 
>>> and dfd + a name gives you fstatat() ?
>> 
>> Yes.
>> 
>>> The cover note could be clearer on this.
>> 
>> Fixed.
>> 
>>> Should the fields really be split the way they are for times rather than
>>> a struct for each one so you can write code generically to handle one of
>>> those rather than having to have a 4 way switch statement all the time.
>> 
>> It depends.  Doing so leaves 16 bytes of hole in the structure.  I could
>> ameliorate the wastage by using a union to overlay useful fields in the gaps,
>> but that's pretty icky and might be compiler dependent.
>> 
>>> Another attribute that would be nice (but migt need some trivial device
>>> layer tweaking) would be STATX_ATTR_VOLATILE for filesystems that will
>>> probably evaporate on a reboot. That's useful information for tools like
>>> installers and also for sanity checking things like backup paths.
>> 
>> There's a FILE_ATTRIBUTE_TEMPORARY that I could map for windows filesystems
>> that could be used with this.
>> 
>>> Remote needs to have clear semantics: is ext4fs over nbd 'remote' for
>>> example ?
>> 
>> Hmmm... Interesting question.  Probably should.  But you could be insane and
>> RAID an nbd and a local disk.  Further, does NFS over a loopback device to
>> nfsd on the same machine qualify as root?  What if that's exposing a local fs
>> on NBD?  Perhaps I should drop 'REMOTE' for now.  It sounds like something
>> that a GUI filemanager might find interesting, though.
> 
> Sorry, I haven't been paying attention, just popping up for this, but:
> "shared" might be a more useful term than "remote".
> 
> A filesystem that may be mounted from more than one system is "shared".
> Caching performance and semantics of such a filesystem are more
> complicated since the filesystem may change out from under us.  This is
> what makes e.g. the lightweight/heavyweight stat difference more
> interesting in the shared case.
> 
> The filesystem should be able to make that shared/unshared distinction
> without knowledge of the storage it's sitting on top of.
> 
> Answering your questions by that criterion:
> 
> 	- ext4/nbd: not shared
> 	- nfs/lo: shared
> 
> But, it's fine with me to drop any features for now as long as we can
> always add them later.

Please, please, please, let's get the syscall and basic functionality
landed first, and then nit-pick about extensions later.  This has been
dragging on for _years_ and bike shedded to death.

Cheers, Andreas






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