Re: [PATCH v4 2/2] hfsplus: validate b-tree node 0 bitmap at mount time

[Shardul Bankar]

On Thu, 2026-02-26 at 23:29 +0000, Viacheslav Dubeyko wrote:
> On Thu, 2026-02-26 at 14:42 +0530, Shardul Bankar wrote:
> > +
> > +       switch (id) {
> > +       case HFSPLUS_EXT_CNID:
> > +               tree_name = "Extents";
> > +               break;
> > +       case HFSPLUS_CAT_CNID:
> > +               tree_name = "Catalog";
> > +               break;
> > +       case HFSPLUS_ATTR_CNID:
> > +               tree_name = "Attributes";
> > +               break;
> > +       default:
> > +               tree_name = "Unknown";
> > +               break;
> > +       }
> 
> Frankly speaking, it could be enough to share only cnid. But if you
> would like
> to be really nice and to share the tree's name, then I prefer to see
> an array of
> constant strings where you can use cnid as an index. And macro or
> static inline
> method that can check cnid as a input argument. At minimum, simply
> move this
> code into the static inline method. But, array of constant strings
> could be much
> compact and elegant solution for my taste. Because, art of
> programming is to
> represent everything as arrays of something and to apply the
> generalized loops.
> :)
> 

Hi Slava,

Sounds good. :) I will implement an array of constant strings indexed
by cnid in v5.

> I prefer not to have the obligation of using this asynchronous
> paradigm of
> kmap_local()/kunmap_local(). It will be great to keep this inside of
> hfs_bmap_get_map_<something>() method.
> 
> I prefer not to keep the whole page/folio for complete operation
> locked. And,
> frankly speaking, you don't need in the whole page because you need a
> byte or
> unsigned long portion of bitmap. So, we can consider likewise
> interface:
> 
> u8 hfs_bmap_get_map_byte(struct hfs_bnode *node, u32 bit_index);
> 
> Here, you simply need to check the state of bit in byte (READ-ONLY
> operation).
> So, you can atomically copy the state of the byte in local variable
> and to check
> the bit state in local variable.
> 

While this byte-level interface is perfect for the mount-time
validation in hfs_btree_open() where we only need to check a single
bit, using it inside hfs_bmap_alloc() introduces a significant
performance regression.

Because hfs_bmap_alloc() performs a linear scan to find a free node,
using hfs_bmap_get_map_byte() inside the while (len) loop would force
the kernel to execute kmap_local_page() and kunmap_local() for every
single byte evaluated (potentially thousands of times per page). The
current logic maps the page once, scans memory linearly, and only
unmaps when crossing a PAGE_SIZE boundary.

To address your request for a generalized map access method without
sacrificing the allocator's O(N) scanning performance, how about this
for v5?

    -We introduce the hfs_bmap_get_map_byte() specifically for single-
bit reads (like the mount-time check). This can internally call
hfs_bmap_get_map_page() from Patch 1/2 to avoid duplicating the offset
math.

    -We retain the page-level helper (hfs_bmap_get_map_page) for
hfs_bmap_alloc() to preserve its fast linear scanning.

Let me know if this dual-helper approach sounds acceptable, and I will
prepare v5.

Thanks,
Shardul