linux-kernel - Re: [GIT PULL] bcachefs

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Message-ID: <20230906231354.GX14420@twin.jikos.cz>
Date:   Thu, 7 Sep 2023 01:13:54 +0200
From:   David Sterba <dsterba@...e.cz>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Kent Overstreet <kent.overstreet@...ux.dev>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-bcachefs@...r.kernel.org
Subject: Re: [GIT PULL] bcachefs

On Wed, Sep 06, 2023 at 06:55:38PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Wed, Sep 06, 2023 at 01:20:59PM -0700, Linus Torvalds escreveu:
> > On Wed, 6 Sept 2023 at 13:02, Linus Torvalds
> > <torvalds@...ux-foundation.org> wrote:
> > >
> > > And guess what happens when you have (unsigned char)-1? It does *not*
> > > cast back to -1.
> > 
> > Side note: again, this may be one of those "it works in practice",
> > because if we have -fshort-enums, I think 'enum
> > btree_node_locked_type' in turn ends up being represented as a 'signed
> > char', because that's the smallest simple type that can fit all those
> > values.
>  
> > I don't think gcc ever uses less than that (ie while a six_lock_type
> > could fit in two bits, it's still going to be considered at least a
> > 8-bit value in practice).
> 
> There are some cases where people stuff the enum into a bitfield, but
> no, no simple type.
> 
> ⬢[acme@...lbox perf-tools-next]$ pahole | grep -w enum | grep :
> 	enum btrfs_rsv_type        type:8;               /*    28:16  4 */
> 	enum btrfs_delayed_item_type type:8;             /*   100: 0  4 */

The simple grep might give a skewed view, in the above case there's also

/* Bitfield combined with previous fields */

in the full output, with adjacent bool struct members it's all packed
into one int. I think I've always seen an int for enums, unless it was
explicitly narrowed in the structure (:8) or by __packed attribute in
the enum definition.

> 	enum kernel_pkey_operation op:8;                 /*    40: 0  4 */
> 	enum integrity_status      ima_file_status:4;    /*    96: 0  4 */
> 	enum integrity_status      ima_mmap_status:4;    /*    96: 4  4 */
> 	enum integrity_status      ima_bprm_status:4;    /*    96: 8  4 */
> 	enum integrity_status      ima_read_status:4;    /*    96:12  4 */
> 	enum integrity_status      ima_creds_status:4;   /*    96:16  4 */
> 	enum integrity_status      evm_status:4;         /*    96:20  4 */
> 	enum fs_context_purpose    purpose:8;            /*   152: 0  4 */
> 	enum fs_context_phase      phase:8;              /*   152: 8  4 */
> 	enum fs_value_type         type:8;               /*     8: 0  4 */
> 	enum sgx_page_type         type:16;              /*     8: 8  4 */
> 	enum nf_hook_ops_type      hook_ops_type:8;      /*    24: 8  4 */
> 		enum resctrl_event_id evtid:8;         /*     0:10  4 */
> 		enum _cache_type   type:5;             /*     0: 0  4 */