Add a function to re-read size of the loop device by vojtechtrefny · Pull Request #1126 · storaged-project/libblockdev

vojtechtrefny · 2025-09-22T07:41:45Z

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gemini-code-assist · 2025-09-22T07:42:02Z

Summary of Changes

Hello @vojtechtrefny, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a critical enhancement to the libblockdev library by adding the bd_loop_set_capacity function. This new API addresses the challenge of dynamically updating the size of an active loop device when its underlying backing file changes. By providing a mechanism to force the kernel to re-evaluate the device's capacity, this change ensures that applications using loop devices can maintain accurate size information without requiring disruptive device re-initialization, thereby improving flexibility and reliability in storage management scenarios.

Highlights

New Functionality: A new function, bd_loop_set_capacity, has been added to the libblockdev library. This function allows users to force the kernel to re-read the size of a loop device's backing file, ensuring the device's reported capacity is up-to-date after the backing file has been resized.
Robust Implementation: The implementation of bd_loop_set_capacity includes error handling for device access and a retry mechanism for the LOOP_SET_CAPACITY ioctl call, specifically handling EAGAIN errors with a short delay.
Comprehensive Testing: A new dedicated test case, LoopTestSetCapacity, has been added to verify the correct behavior of the new function. This test simulates resizing a backing file and confirms that bd_loop_set_capacity successfully updates the loop device's reported size.
API and Documentation Updates: The new function has been properly declared in the loop.api and loop.h files with Doxygen comments, and its entry has been added to the libblockdev-sections.txt documentation index.

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Code Review

This pull request introduces a new function bd_loop_set_capacity to allow re-reading the size of a loop device, which is useful when the backing file has been resized. The implementation includes the core logic, API definitions, documentation updates, and a new test case.

My review has identified a bug in the progress message formatting that could lead to incorrect paths in log messages. I've also provided suggestions to improve code readability in the C implementation and reduce code duplication in the Python test suite. Overall, the changes are well-structured and the new functionality is a valuable addition.

vojtechtrefny · 2025-09-22T08:02:50Z

/gemini review

gemini-code-assist

Code Review

This pull request adds a new function, bd_loop_set_capacity, which allows forcing a loop device to re-read the size of its backing file. The implementation is sound, and it's accompanied by good documentation and a comprehensive test case. The changes are well-structured. I have one suggestion to improve code readability in the C implementation.

gemini-code-assist · 2025-09-22T08:04:58Z

+    for (n_try=10, status=-1; (status != 0) && (n_try > 0); n_try--) {
+        status = ioctl (fd, LOOP_SET_CAPACITY, 0);
+        if (status < 0 && errno == EAGAIN)
+            g_usleep (100 * 1000); /* microseconds */
+        else
+            break;
+    }


The logic in this for loop is a bit dense due to the combined initialization and condition, along with the if/else break structure. A more conventional loop structure would be more readable and easier for future maintenance.

for (n_try = 10; n_try > 0; n_try--) { status = ioctl (fd, LOOP_SET_CAPACITY, 0); if (status == 0) break; if (errno != EAGAIN) break; g_usleep (100 * 1000); /* microseconds */ }

I like this better!

tbzatek · 2025-09-23T08:45:55Z

+    for (n_try=10, status=-1; (status != 0) && (n_try > 0); n_try--) {
+        status = ioctl (fd, LOOP_SET_CAPACITY, 0);
+        if (status < 0 && errno == EAGAIN)
+            g_usleep (100 * 1000); /* microseconds */
+        else
+            break;
+    }


I like this better!

tbzatek · 2025-09-23T08:48:31Z

+    g_free (dev_loop);
+    if (fd < 0) {
+        g_set_error (&l_error, BD_LOOP_ERROR, BD_LOOP_ERROR_DEVICE,
+                     "Failed to open device %s: %m", loop);


So previously you use a complex construct like dev_loop ? dev_loop : loop for messages but in the rest of the code you just use loop... I don't mind, I wonder why the AI didn't spot this.

Because I copied it from two different functions with two different messages :-)

gemini-code-assist Bot reviewed Sep 22, 2025

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Comment thread src/plugins/loop.c Outdated

Comment thread src/plugins/loop.c

Comment thread tests/loop_test.py Outdated

Add a function to re-read size of the loop device

f191bf8

vojtechtrefny force-pushed the master_loop-set-capacity branch from 9df1103 to f191bf8 Compare September 22, 2025 07:48

gemini-code-assist Bot reviewed Sep 22, 2025

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vojtechtrefny mentioned this pull request Sep 22, 2025

Add a function to re-read loop device capacity storaged-project/udisks#1422

Merged

tbzatek approved these changes Sep 23, 2025

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vojtechtrefny merged commit 24f99d3 into storaged-project:master Sep 23, 2025
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Add a function to re-read size of the loop device#1126

Add a function to re-read size of the loop device#1126
vojtechtrefny merged 1 commit into
storaged-project:masterfrom
vojtechtrefny:master_loop-set-capacity

vojtechtrefny commented Sep 22, 2025

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vojtechtrefny commented Sep 22, 2025

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