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Subjectminixfs question

Let me apologize in advance for any ignorance I might exhibit here. I am
curious about the following code, from minix_new_inode in fs/minix/bitmap.c.
The if statement on line 232 checks !bh. It seems that if bh was null at this
point, then sbi->s_imap_blocks == 0, in which case we fail the other half of
the if. If bh == NULL in any other case, wouldn't the bh->b_data in the
minix_find_first_zero_bit call be deref. a NULL pointer, which would trigger a
kernel panic or some such terminal event? Also, why does at least this
filesystem use minix specific version of these bitmap operations? Isn't there
a generic set of these operations? In other functions, it seems like these
tests have to be protected by the BKL? Isn't the point of things like
test_and_set and test_and_clear that they are atomic? If so, why the need for
the BKL (for example, minix_free_block)? Also I noticed this these files seem
to have very sparse commenting, would a patch consisting of germane commenting
for this fs be welcome? Also, would it be beneficial if this fs was converted
to use one or more minixfs specific locks instead of the BKL?

223 j = 8192;
224 bh = NULL;
225 *error = -ENOSPC;
226 lock_kernel();
227 for (i = 0; i < sbi->s_imap_blocks; i++) {
228 bh = sbi->s_imap[i];
229 if ((j = minix_find_first_zero_bit(bh->b_data, 8192)) <
230 break;
231 }
232 if (!bh || j >= 8192) {
233 unlock_kernel();
234 iput(inode);
235 return NULL;
236 }
237 if (minix_test_and_set_bit(j,bh->b_data)) { /* shouldn't happen
238 printk("new_inode: bit already set");
239 unlock_kernel();
240 iput(inode);
241 return NULL;
242 }
243 unlock_kernel();

Erik McKee
*Who is hoping this fs is a good place to start learning about fs in linux.*
Please cc me in replies, as I am not on list.

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